functionalizing polymer surfaces by field-induced migration of copolymer additives—role of shear...

Functionaliting Polymer Surfaces by Fie Id4 nduced Migration

of Copolymer Additives-Role of Shear Fields

HOJUN LEE' and LYNDEN A. ARCHER2*

'Department of Chemical Engineering Texas A&M University

CoZlege Station, 'Tx 77843

2School of Chemical Engineering Cornell University

I t h a , NY 14853-5201

Flow-induced migration polyethylene-co-methacrylic acid (PE-co-MA) and polystyrene-b-polydimethylsiloxane (PS-b-PD MS) copolymer additives in commercial long-chain branch polyethylene (PE) and narrow-molecular- distribution polystyrene (PS) hosts was investigated in a capillary flow device. Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy and Dynamic Contact Angle (DCA) measurements were used to characterize surface composition of polymer specimen following extrusion through metallic dies with various length-to-diameter (L/D) ratios, (1100 5 L / D 5 3000). Results from experiments covering a broad range of shear rates and polymer residence times in the dies are reported. Provided that the polymer residence time in the die is sufficiently long, shear is found to increase the concentrations of low molecular weight copolymer additives on the host polymer's surface. The surface composition of copolymer additive is found to vary strongly with the wall shear rate and die L / D ratio. Decreasing the die diameter at fixed flow rate is found, for example, to be a more effective method for enhancing transport of additive to a polymer's surface than increasing shear rate at fixed diameter. A mechanism based on shear-induced diffusion is proposed to explain the observed migration.

INTRODUCTION

t has long been known that low molecular weight I additives initially homogeneously dispersed in a viscous polymer host might migrate to the surface of the host polymer under the action of shear in melt processing equipment. This process is, for example, widely believed to be the mechanism by which slip- promoting soaps migrate to polymer/die interfaces during extrusion processing to reduce operating pressure requirements for thermoplastics processing (1). If harnessed, such migration could provide a powerful method for transporting other surface functionalizing species to polymer surfaces during normal melt processing. The method is particularly attractive because it relies on physical processes to modify the chemical make-up of polymer surfaces and therefore does not

*Corresponding author.

require extra processing steps or new equipment. In addition, flow-induced migration can in principle be used to impart any surface specific property (e.g. ad- hesion, paintability, biocompatibility, and lubricity) not present in bulk. Technological progress in the area has been impeded by a lack of fundamental understanding of the migration process itself. Indeed, even though the practice of introducing slip additives to polymers has been around for years, formulation of additive packages based on these materials remains largely a matter of trial and error experimentation.

Fundamental studies of additives migration in polymers could also prove useful for preventing migration of non-surface-active additives (e.g. plasticizers and colorants) during processing and in service. Such studies could, for example, be used to guide design of such additives to ensure they remain dispersed in the bulk polymer during processing, as well as in service. Our previous two papers in this series (2. 3) have discussed the quiescent (no-flow) surface migration of

1568 POLYMER ENGINEERING AND SCIENCE, JULY 2002, Vol. 42, No. 7

f inc t ional i z ing Polymer Surfaces by Field-Induced Migration of Copolymer Additives

polystyrene-b-polydimethylsiloxane (PS-b-PDMS) and polystyrene-b-poly (methyl methacrylate) (PS-b-PMMA) copolymer additives in narrow-molecular weight distribution polystyrene hosts. This work demonstrated that differences in surface energy and/or molecular weight between copolymer additives and host polymer materials play important roles in determining the di- rection and amount of additive migration in the host polymer. Additionally, the interaction between polymer and a bounding solid wall was also shown to significantly contribute to the migration of polymer in chemically dissimilar blends.

Flow-induced migration of low molecular weight and polymeric species in dilute polymer solutions has been studied extensively in the literature (4-9). Shafer et al. (4) and Dill and Zimm (5). for example, reported modest levels of radial migration of DNA molecules in dilute aqueous solution towards the center of a cone- and-plate flow device. The authors measured the radial polymer concentration profile after shearing for several hours and found that DNA chains migrate at a rate of about 3 cm in 3 hours. Shafer et aL (4) argued that radial polymer migration was driven by an elas- tic normal force component due to the curvature of streamlines in cone-and-plate shear. Based on this idea Dill and Zimm (5) proposed an empirical expres- sion for the radial migration velocity of polymer in dilute solution during cone-and-plate shear flow:

where r is the radial position, R, is the average un- disturbed end-to-end distance of polymer chain, qs is the solvent viscosity, and kB is Boltzmann's constant. This result implies that the rate of migration should be rather strong functions of polymer molecular weight and shear rate. More recently, McDonald and Muller (6) reported large levels of radial migration of high molecular weight polystyrene in dilute solutions during shear in a cone-and-plate geometry. The authors compared their results with predictions based on the two-fluid theory for stress-induced migration (7) and found substantial discrepancies between observed and predicted migration levels, suggesting that factors other than the curvature of streamlines are important.

Other theories for polymer migration in dilute solution contend that inhomogeneous distributions of de- formation rate, stress, and/or molecular configuration near solid walls are required for migration in solution (8-12). Specifically, long polymer chains in high-stress regions of a flow field (e.g. near the die walls in a cap- lllary extruder) are thought to experience much larger reductions in configuration entropy than smaller molecules. This creates a thermodynamic incentive for migration of higher molecular weight species away from die walls and for countercurrent migration of lower molecular weight species to these same walls during flow. Thus, in polymer solutions a nonhomoge- neous flow field depletes polymer molecules from high

stress regions near the surface and enhances the concentration of low viscosity solvent tlhere. Implications of this phenomena on apparent slip violations in polymer solutions have been discussed by Drout and Ber- rajaa (10). Other migration mechanisms based on flow enhancement of the apparent mutual diffusion coefficient of an additive in the host polymer (11) and on flow-induced changes in the additive's miscibility its polymer host (7, 12) have also been suggested.

Provided sufficient time is allowed for diffusion to the surface during flow, a low molecular weight polymer component in a polymer blend1 could migrate to the material's surface by any of the mechanisms discussed. Schreiber et al. (13, 14). for example, reported molecular weight fractionation in broad molecular weight distribution (MWD) polyethylene melts in capillary dies that appears to fo'llow this mechanism. The authors used static laser light scattering measurements to quantfy spatial variations in molecular weight of sectioned polymer e h d a t e s . The extrudate skin was found to be of lower molecular weight than its core, supporting the idea that low molecular weight components in the polymer mi- grated to its surface during flow. This hypothesis was further confirmed by the fact that reductions in average skin molecular weight were more pronounced with large (L/D) dies. Whitlock and Porter (15) studied fractionation of lower molecular weight components in polydisperse polystyrene melts using a different molecular weight analysis technique. These authors did not observe sigmficant molecular weight fractionation, suggesting that the melt migration results of Schreiber et d were perhaps not general.

The objective of the present study is to investigate flow-induced migration of copolymer additives in high molecular weight polymer hosts. Our goal is to de- velop a detailed understanding of ithe mechanism by which the phenomenon occurs in commercially important polymers (high molecular weight melts and concentrated solutions). We believe that a key limitation in earlier work is that insufficient shearing time was allowed for additive transport in the viscous polymer host under the applied flow. This limitation is ad- dressed in the present study thro~tgh use of metallic dies with a very wide range of L/D ratios. We also pay attention to separate effects of die gap on diffusive surface transport and shear rate.

EXPERIMENT

Materials

Narrow molecular weight distrib'ution (MWD) poly- styrenes with a wide range of molecular weights and commercial long-chain branch polyethylenes with a range of melt indices were used in the study. Poly- styrenes were purchased from Polymer Source and Aldrich Chemical Company, while the polyethylenes were donated by Exxon Chemical Company. Broad MWD polyethylene-co-methacrylic acid (PE-co-MA) and narrow MWD polystyrene-co-dimethyl siloxane

POLYMER ENGINEERING AND SCIENCE, JULY 2002, Vol. 42, No. 7 1569

Hojun Lee and Lynden A. Archer

(PS-co-PDMS) copolymer additives were purchased from Aldrich and Polymer Source, respectively. The chemical composition, molecular weight, and methods for producing homogeneous blends of the latter materials with PS hosts have been described previously (2). PE-co-MA copolymers used in the study contained - 10 wtYo MA and possessed melt flow index of 500 g/min at 190°C.

PE/PE-co-MA blends containing 10% copolymer additive (90/10 PE/PE-co-MA) were prepared by solution casting and/or melt mixing using a twin-screw extruder. Solution cast films were prepared by dissolv- ing the polymer components in the required propor- tions in xylene at 120°C (2 wtYo polymer in solution). Subsequent evaporation of the xylene at 40°C in alu- minum pans yielded sample films with controlled thickness in the range of 40-50 km. At the solution preparation conditions, xylene is a good solvent for both PE and PE-co-MA, so selective surface enrichment due to differential miscibility of the polymer components was minimized during film casting. The remaining traces of solvent were removed in a final vacuum evaporation step at room temperature.

prior to the capillary flow experiments, rheological properties of all polyethylene samples were character- ized by small-amplitude oscillatory shear and steady shear flow measurements using a Paar Physica Uni- versal Dynamic Spectrometer (UDS), equipped with

1 o6

1 o5

1 o4

1 o3

UU O0

stainless steel parallel plate fixtures (25 mm diameter, 300 krn gap). Typical frequency-dependent storage and loss moduli for PE are shown in Fig. 1. Also shown are the complex viscosity and steady shear viscosity versus frequency and shear rate, respectively. The longest re- laxation times were estimated from the zero-shear viscosity and steady-state recoverable creep compliance at low flow rates and stresses, respectively. Rheologi- cal properties of interest for polyethylene matrix polymers are listed in Table 1 .

Methods

Attenuated total reflection Fourier Transform Lnfi-ared spectroscopy (ATR-FTIR) and dynamic contact angle (DCA) measurements were used to characterize the surface properties of copolymer additives in polymeric hosts. As in our previous work on additives migation in polymers (2, 3), ATR-mR spectra were calibrated using pure copolymer with known concentration of methacrylic acid or dimethyl siloxane groups, allowing quantitative surface composition information to be recovered from ATR-FTIR measurements. In every case calibration curves determined in this manner were found to be nonlinear functions of composition, ex- cept at very low compositions. ATR FTIR absorbance spectra for PE/PE-co-MA are provided in Fig. 2. The absorption peak at - 1750 - 1650 cm-1 is easily

0 OO

OO

0.1 1 10 100 1000

o (rad s'l) m. 1 , Frequency-dependent storage G and loss G" modulifor PE-01 (Iw = 1 .O) at 175°C. Also shown are the dynamic and steady- shear viscosities of PE-01 versusfiequency and shear rate, respectbz-ly. Measurement were performed using a torsional shear rheometer equipped with 25.4-m-diameter stainless steel plates.


Functionalizing Polymer Surfaces by Field-Induced Migration of Copolymer Additiues

Table 1. Rheological Characteristics of Matrix Polyethylene Measured at 175°C.

Polymer Melt index ?.,(Pa s) J, h h o x J,) PE-01 1 .o 60800 1.2 X lo-" 7.6 PE-03 3.0 2818 1.4 X lo4 2.9 PE-10 10.0 2050 5.2 X lo-" I .07

assigned to the asymmetric stretching vibration of C = O bonds in the methacrylic acid groups, while the absorption band at - 1465 cm-' has been assigned to vibrations of C-H bonds present in the host polymer and copolymer additive. Surface concentrations of the PE-co-MA additive were therefore calculated using the ratio of the two peak heights, i.e. the C-H vibration was used as an internal reference for quantitative analysis.

Extrusion experiments were conducted at a various temperatures (150-190°C) using a capillary rheometer designed to be operated either at constant speed or constant extrusion force. A schematic diagram of this instrument is provided in F7g. 3. Extrusion experiments were performed under constant force conditions by driving the hydraulic cylinder with an inlet air stream at fixed pressure. Constant speed extrusion measurements were performed using a motorized con- trol valve to regulate the flow of oil into the hydraulic cylinder barrel. The extrusion rate and extrusion force were measured independently using a calibrated

Linear Variable Differential TransfoImer (LVDT) and a load cell, respectively. Capillary dies with lengtt-to-diameter (L/D) ratios in the range :LOO to 3000 were used in the study.

RESULTS AND DISCUSSION

Quiescent Migration

As discussed in our previous articles (2, 3). copolymer additives homogeneously dispersed at low concentration in a polymer host can migrate to the host polymer surfaces under the action 'of dominantly enthalpic (surface energetic) or dominantly entropic (configurational entropy gradient) driving forces. These migration mechanisms are, by definition, com- mon to all polymer/copolymer additive mixtures. Thus, to isolate the effect of an external flow field on the surface composition of a polymer, it is essential that migration experiments under quiescent conditions be performed. Here we focus 011 results obtained using the system PE/PE-co-MA, the corresponding data for PS/PS-co-PDMS have been thoroughly described in reference 2. The system PE/PE-co-MA is particularly interesting because the surface tension of the PE and PE-co-MA materials used in the study are for all practical purposes identical (-30 mN m-l). Therefore, the primary driving force for spontaneous (quiescent) surface enrichment by either material is likely entropic.

I -Surface I

i 1 M Bulk

500 1000 1500 2000 2500

Wavenumber (cm-') Fig. 2. Am-FilR spectra for the surface and buUc of a PE-Ol/PE-co-MA blend extruded at a nominal wall shear rate Tw of 0.1 s-' using a cap- die (L/D = lo2). Bulk samples were sectionedfrorn the centerline of the extrudate at the same rucial location where the surface chemical composition was evaluated using Am-FIZR.

POLYMER ENGINEERING AND SCIENCE, JULY2002, Vol. 42, No. 7 1571


Compression Air Motorized flow

Hydraulic Fluid

& Barrel support

U

Fig. 3. Schematic diagram of the hybridp- ' /hydraulic driven ewbuswn apparatus used in the study. This instrument can be used to perform extrusion measurements under controlled force and controlled rate conditions.

The surface concentration of PE-co-MA in PE/PE- co-MA blends as a function of annealing time at a temperature of 170°C is summarized in Fig. 4 for three different polyethylenes. Here and throughout the paper, surface concentrations are reported in weight percentage of the copolymer additive. I t is readily apparent from these results that the surface concentration of PE-co-MA strongly depends on both annealing time and viscosity of the host polyethylene. It is also apparent from Fig. 4 that provided sufficient annealing time is allowed at elevated temperatures, substantial levels of surface migration can and will occur in the system PE/PE-co-MA even in the absence of a flow field. The surface concentration of PE- co-MA is nonetheless observed to increase roughly as the square root of annealing time for each of the three polymer/additive systems. This behavior clearly implies that transport of PE-co-MA to the host polymer surface is diffusion limited, and that the diffusion process is fairly well described by Ficks law. At short times and for thin diffusion films such as those probed by our ATR-FIIR measurements, the slope of the concentration versus 6 plot is directly re- lated to mutual diffusion coefficient DPE/Copolymer, of the copolymer additive in the host material. Figure 4 in fact shows that the slope decreases with decreasing melt index of the polyethylene matrix, which is consistent with a decrease in D,/,,,l,,, as the viscosity of the polymer host increases.

Flow-Induced Migration of PE-CO-IYLA in PE

Next we turn to considering the effect of an external flow field on surface composition of PE/PE-co-MA blends. For this purpose, ATR-FTIR spectra of various 10/90 (PE-co-MA/PE) blends were collected following extrusion over a range of wall shear rates, extrusion forces, and using dies with a variety of L/D ratios. These measurements were supplemented with ATR-FTIR measurements of center-sectioned speci- mens of the extruded samples, to determine the bulk composition of PE/PE-co-MA extrudates. At low extrusion rates, the ratio of the C=O peak height to the C-H peak height at the surface of the extrudate was found to be significantly higher than that of the bulk polymer harvested at the same axial location (see Fig. 2). These spectra lend qualitative support to the idea that the low molecular weight PE-co-MA preferen- tially segregates to the surface of the higher molecular weight PE host under the action of shear in the die.

Additional support of this finding is provided in Hg. 5 4 where ATR-FlTR spectra of PE/PE-co-MA blends extruded at fixed rate, but at various die gaps are pre- sented. It is clear from the Figure that the C=O peak height increases significantly as the die gap is lowered while holding the extrusion rate constant. Quantitative confirmation that PE-co-MA migrates to the surface of PE/PE-co-MA blends is also provided in Fig. 5b. This


Functionalizing Polymer Surfaces by Field-lnduced Migration of Copolymer Additives

40

2 30- 0

w Y

-42 20

10

1- -

-

-

U PE-01

01 , 1.5 2.5

m. 4. Time-dependent sut fae concentration of PE-co-MA (in weight percent) at the air/polymer interface following onset of thermal annealing at 170°C.

figure also summarizes the effect of wall shear rate on surface excess of PE-co-MA ( A 4 = +surface- + b ~ ) for four different residence times of polymer in the die. PE/PE-co-MA blends used in this portion of the study were prepared using a twin-screw extruder that facili- tates separate feeding of PE and copolymer additive. ATR-FTIR measurements on blend samples prior to these measurements revealed completely homogeneous distribution of host polymer and copolymer additive. Flgure 5b clearly shows that PE-co-MA copolymer concentrations at the surface are higher than in the bulk over the entire range of shear rate studied. It is also evident from the results that the enhancement of PE-co-MA produced by flow is substantially larger than expected under quiescent conditions for the same annealing (residence) time. A second effect of polymer residence time in the die on surface migration levels is also readily apparent from the data. At the lowest residence times studied, an approximate relationship A 4 - 7”s can be defined to summarize the effect of wall shear rate,

on surface excess PE-co-MA. An even stronger effect A4-7 lZi s evident in materials sheared for longer times in the die. Both results point to the importance of copolymer additive diffusion for surface functionalization of polymer hosts. Thus, even though the driving force for migration can be large, if insufficient

residence time is allowed in the die, only minimal changes in surface concentration will result.

For large residence times, the effect of wall shear rate on surface excess PE-co-MA is clearly stronger than the effect of shear rate on stress in the Newton- ian melt flow regime. This finding suggests that the shear stress conditions in a die is pel-haps not the driving force for surface migration in a polymer/additive blend. To probe the origin of this behavior further, rheological behaviors of PE/PE-co-MA were investigated under conditions similar to those at which flow- induced migration in these systems were investigated. To this end, the extrusion force F at ,various extrusion rates was measured during the start-up of controlled speed extrusion. Wall shear stress can be estimated (i.e. without correcting for entrance and exit losses) from the measured extrusion force using the relation,

7, = R F / P A L . (3)

Here A is cross-section area and R imd L are die ra- dius and length, respectively.

Typical time-dependent wall shear stresses obtained in this manner are provided in Fig. 6a All features of the data, including exponential stress growth and relax- ation, and slower than linear increase of steady-state shear stress with increasing shear rate are well known for pseudoplastic polymer liquids. Steady-state wall shear stresses obtained from these plots for variable wall shear rate are summarized in F’ig. 6b for two die



Die Gap (mm) 0. i 0.25 0.5

0.8 1 .o

700 1000 1300 1600 I900 2200

Wavenumber (cm- ' ) (4

0.1 /, . / '

2500

,

0.01 0.1 I 10 100

(b) Fig. 5. (a): ATR-FI7R spectra of 90/10 PE/PE-co-MA blends extruded at afixed rate of 0.042 mm/s and at a temperature of 175°C using various extruder die gaps. (b): Surface excess concentration of PE-co-h4A (in weight percent) on PE/PE-co-UA blend (MIpE = 10.0) as afunction of wall shear rate for four separate shearing times.

1574 POLYMER ENGINEERING AND SCIENCE, JULY 2002, Vd. 42, No . 7

Functionalizing Polymer Surfaces by Field-Induced Migration of Copolymer Additives

4 ,

I I I

= i I I

A 0.085 mm/s

0 0.170mds

o 0.34Omm/s

o 0.890mds

0 50 100 150 200

Time (s.)

(a)

0 101.6 mm.

50.8 mm.

0.01 10

Fig. 6. (a): Wall shear stress as afunction of time for a 90/10 PE/PE-w-MA blend &flm = 10.0) ex6uded at 175°C through a 5 - m - long (0.1 -mm-gap) die at various evbuswn rates: (b] Steady state wall shear stress T, versus wall shear rate qmjor the same 90/10 PE/PE-cc-MA blend at two die lengths.


ffojun Lee and Lynden A. Archer

lengths. These results do show an effect of die length on the stress-shear rate diagram. Specifically, as the die length increases, the apparent viscosity qa = T,/+, of the polymer is higher at all wall shear rates, but increases more slowly with increasing shear rate. This effect can conceivably arise from depletion of the low viscosity additive from the bulk material by migration, but its origin is not yet known. Independent steady-shear cone-and-plate rheological measurements using the UDS support a relationship of the form 7 - +0.7 between shear stress and shear rate. It is therefore apparent that wall shear stress in the PE/PE- co-MA systems studied increases more slowly (roughly by a factor of order +,) with wall shear rate than the surface excess copolymer additive concentration at the host polymer surface. Relationships between surface excess additive and divergence of the stress tensor. A+ - V . T , (7, 12) or between A+ and first normal stress difference in shear, A+ - N , , are both consistent with the experimental observations.

Migration is driven in the first situation, by higher stresses at the die surface than in the center. Such a stress distribution favors a higher concentration of the highest molecular weight species in a blend at the center of the die, where the shear stress is a mini- mum. In an incompressible polymer/additive blend, inward migration of the higher molecular weight polymer host produces countercurrent transport of the additive to the die surface, in agreement with our observations. In the second situation, migration occurs by a pressure-induced transport of the less viscous copolymer component in the blend to the surface of the more viscous host polymer. Again, the conse- quence of migration is enhancement of the additive concentration at the extruded blend surface. At low extrusion rates (+, Q A-6). the driving force for such transport is anticipated to be of order Nl - +:, which is not too far from the experimental observations. Un- fortunately, our current extrusion system is incapable of providing Nl measurements in the system studied, making it impossible to differentiate between migration by either of the two mechanisms. Work is under way to interface a laser-birefringence optical polarime- ter with the current extrusion system to facilitate real time local measurements of Nl and T in polymer/additive blend extrusion experiments between a metallic and high refractive index glass substrate. These measurements will allow us to simultaneously quan- tify the effect of shear rate on N, and on shear stress gradients within 100 nm from the glass substrate, permitting the migration mechanism to be studied in more detail.

Flow-Induced Migration of PS-b-PDMS Copolymer Additives in PS

The similarity of the surface tension of pure PE and PE-co-MA implies that the surface energy of the die material should have little, if any, effect on additives migration during extrusion. This situation changes

considerably in the system PS/PS-b-PDMS because of the much lower surface energy of DMS groups in the copolymer additive (2). Specifically, in this system it is considerably more favorable for DMS groups to con- centrate at phase interfaces with low energy materials (e.g. air). Thus, the mechanical driving force needed to produce PS-b-PDMS additive migration to a metallic die surface during extrusion of PS/PS-b-PDMS blends is anticipated to be rather large.

To investigate migration of PS-b-PDMS copolymers in PS, blends containing 90 Woh polystyrene (%, = 2 x lo6 glmol, PI = 1.02) and a diblock copolymer of polystyrene M, = 193 X lo3 g/(moZ) and poly dimethyl siloxane (M, = 39 x lo3 g/(mol)), PI = 1.011 were prepared by precipitation from 2 Woh toluene solution using methanol (2). We have shown previously that in the absence of high-temperature thermal annealing, materials prepared using this procedure contain a homogeneous distribution of PS-b-PDMS in PS (2). These materials were extruded at 185°C through a long stainless/steel cylindrical die [L/D = 3 X lo3) at a variety of pressures. Controlled pressure extrusions were chosen in this case primarily based on two con- siderations. First, the large transient pressure drops required to extrude the material at constant rate sur- passed the die/barrel seal operating pressure range; Second, because of the slow transport of the high molecular weight PS-b-PDMS copolymer additive in PS, very long shearing times are required for diffusive transport of additive to the host polymer surface.

Surface and bulk (by sectioning) chemical make-up of polymer extrudates created using these procedure were analyzed using water contact angle and ATR- FTIR measurements. Typical ATR-FI'IR spectra of PS/PS-b-PDMS extrudates are shown in Fig. 7. The figure clearly shows that the characteristic DMS peak at -1250 cm-' is significantly enhanced relative to that of the bulk, indicating that DMS groups can be induced to segregate near a high energy metallic interface under the action of shear. The large penetration depth of ATR-FTIR (approximately 0.5 mm, for the materials studied here) raises questions about the structure of the PS-b-PDMS copolymer at the interface (e.g., are the DMS groups at the surface or buried beneath a thin PS undercoat?). Contact angle measurements provide a simple way of answering this question because they characterize surface atomic composition with a few A of an interface. For this purpose a Wilhelmy plate dynamic contact angle procedure (2) was used to determine the water contact angle of PS/PS-b-PDMS specimen extruded at a variety of wall shear stresses. Figure 8 shows that the contact angle decreases as the wall shear stress increases. This curious behavior would appear to imply, at least naively, that the concentration of DMS groups within a few Angstroms of the polymer/die interface, though higher than in the quiescent material, decreases as the intensity of shearing increases. Shorter shearing times (residence time in the die) at the higher wall shear stresses (i.e. extrusion pressures)


Functionalizing PoZymer Surfaces by Field-Induced Migration of Copolymer Additives

- Surface

1

850 1250 1650

Wavenumber (cm-') Flg. 7 . ATR-Fl7R spectra ofpS/PS-bPDMS (90/10] blend before and after ewtruswn at 185°C through a long stainiless/steel capluary die (L/D = 3 X l@).

130

5 100

85

+ Advancing

rn Recoding

0 20 40 60 80 100

2, x 1 0 - ~ pa] FXg. 8. Advancing and receding water contact angle ~ S L L S wall shear stress for a PS/PSb-PDMS specimen extruded at 185°C.



appear to be the most likely cause of this observation, underscoring the importance of molecular diffusion in migration even in the presence of shear.

Measurable levels of contact angle hysteresis are also evident from the data, suggesting that the surfaces of extruded materials are quite dynamic. Surface composition of DMS groups was determined from advancing contact angle measurements on PS/PS-b-PDMS extrudates and contact angle measurements using pure PDMS films deposited on glass substrates. DMS surface compositions obtained in this manner were supplemented with surface compositions determined using calibrated (2, 3) ATR-FTIR measurements on the polymer extrudates. Results obtained using both procedures are provided in Q. 9 at various wall shear stresses. It is immediately apparent from this figure that the concentration measured by ATR-FTIR is much lower than that by contact angle measurements. I t is also apparent that the contact angle de- rived data are much more sensitive to changes in extrusion conditions. These differences are consistent with previous observations from quiescent migration studies of PS-b-PDMS (2) and PS-b-PMMA in PS, and arise from the large difference in interface depth probed by the two methods. Specifically, the larger penetration depth of the ATR-FTIR measurements means that the surface composition recovered is aver- aged over a much larger interface thickness than is the corresponding information recovered from contact angle measurements. This in turn means that diffusion

limitations to transport have a much larger effect on “surface” composition measurements obtained using the former technique.

CONCLUSIONS

Flow-induced migration of polyethylene-co-methacrylic acid in polyethylene and polystyrene-b-poly dimethyl siloxane in polystyrene are investigated using Attenuated Total Reflection Fourier Transform Infrared spectroscopy and Dynamic Contact Angle measurements. Flow was generated using a novel extrusion system capable of producing true controlled rate and controlled force deformations in polymer melts. Selective enrichment of copolymer additives at the host polymer surface was observed in both polymer systems studied. In the system PE/PE-co-MA, surface excess concentration of PE-co-MA additives, A+, was found to depend on wall shear rate, shearing time, PE melt flow index, and die geometry. Specifi- cally, as shearing time (polymer residence time in the die) was increased, A+ manifested progressively stronger dependence on wall shear rate p,, eventually showing a dependence A+ - 9’;” for long shearing times (die lengths). This finding is believed to under- score the importance of molecular diffusion in polymer surface functionalization processes based on flow-induced migration. The relationship observed between A+ and 4, for long shearing times also points to a rather strong dependence of the driving force for

60

2 Q k ’ 40 9

I m k

8.

20

0

+ contact angle

-o- ATR-FTIR

0 20 40 60 80 100

TW [MPaI m. 9. Comparison of surf= concentraiion of PS/PS-b-PDMS exbudates calculated from contact angle data and from ATR-FIIR measurements. Both measurements were perfarmed OR 9011 0 blend samples extruded at 185°C and constant pressure.

I 578 POLYMER ENGINEERING AND SCIENCE, JULY 2002, Vol. 42, No. 7

finctwnalizing Polymer Surfaces by Field-Induced Migration of Copolymer Additives

migration in polymer melts on shearing conditions. Indeed, a dependence of the form A+ - y’;” rules out the shear stress as the driving force for migration, but is consistent with a migration process that depends on divergence of the stress tensor, A+ - V * 7, or on the first normal stress difference in flow, A+ - Nl. There are two main practical implications of our observations in the system PE/PE-COMA. First, the results demonstrate that if sufficiently long residence time is allowed in the die, flow-induced migration can be used to functionalize polymer surfaces during normal melt processing. The length of the residence time required for migration in any given system may be com- puted, provided the mutual diffusion coefficient of the functionalizing additive in the polymer host is known. Second, our results indicate that the driving force for migration is likely largest in flow geometries where there are significant shear stress gradients normal to the surface to be functionalized and/or in polymer systems with large normal stress differences in shear (high molecular weight, entangled polymers).

Flow-induced migration of relatively high molecular weight PS193k-b-PDMS 39k copolymer additives in a high molecular weight PS (M, = 2 X lo6 g /mol ) host was also studied under controlled stress extrusion conditions. This system is interesting because DMS groups interact repulsively with high-energy metallic substrates. An enthalpic driving force therefore exists for copolymer additive migration away from the surface. Despite this, ATR-FTIR and Dynamic Contact Angle measurements on extruded tubes of PS/PS-b- PDMS reveal rather large levels of DMS at the surface at all wall shear stresses studied. For this material, however, the concentration of DMS units at the surface was observed to decrease with increasing shear stress, a finding believed to arise from lower residence

times at high stresses. This trend can be reversed by increasing the L/D ratio of the die used, or by using much lower molecular weight additives for surface functionalization.

ACKNOWLEDGMENTS

The authors thank Equistar and Exxon Chemicals for providing polyethylene samples. Financial support from NSF Career program, Texas Higher Education Coordinating Board Advanced Research program, and from 3M Corporation is gratefully acknowledged.

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functionalizing polymer surfaces by field-induced migration of copolymer additives—role of shear...

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