Indian Jou rnal o f Pure & Applied Physics Vol. 40, May 2002, pp. 36 1-366

X-ray crystallographic studies of polymeric materials N V Bhat & R R Deshmukh

Department of Chemical Technology, University of Mumbai , Matunga, Mumbai 400019

Received 24 July 200 I ; revised 22 February 2002; accepted 18 March 2002

X-ray diffraction (XRD) studies are very usefu l for distinguishing crystalline and non-crystalline states. Highl y crystal line substances give patterns with sharp diffraction peaks or spots, whereas liquids or amorphous material s lie between these two extremes, because their chain length is large and it is difficult to attain three-dimensional regularity. However, XRD stud ies of polymers yield interesting results. A large number of polymeric materials such as po lyeth ylene, polypropylene, nylon, polyethylene terephthalate (PET), polyvinylidine tluoride etc. as well as natural polymers like, cotton and silk have been studied. The diffraction pattern usually consists of some sharp peaks together with broad halos. A carefu l analysis of the diffraction pattern allows us to find out the crystallite shapes , sizes and percentage crysta llinity. The diffraction pattern is very sensiti ve to thermal hi story, chemical processing and other fac tors. In the present paper, the results of XRD of PET when subjected to thermal , chemical and plasma treatments have been presented . The results show that the percentage cryst allinity can be varied from 17 to 50 %.

1 Introduction

Polymers are a new class of materi als and occupy an important place in our every day life . Plastics are used extensi vely from household artic les to aircraft and space vehicles in the form of different components '. The growth of material science has been continuous from metals, a lloys , semiconductors and ceramics . These material s find use in several applications as their structures and properties have been investigated over last one hundred years. Plastics are comparati vely new entrants having a very complex structure and have been investi gated only for last about forty years. However, the progress in the area has been rapid , mainl y, due to combined efforts of chemists, physicists, engi neers, technologists and material sc ientists.

On account of these complexities invo lved in its structure, from atomic and molecu lar level to the bulk , it is essenti al to study the properties using a varie ty of techniques. Commerc ia ll y, polyethylene (PE), po lypropy lene (PP) , nylon, polyethylene­terephth alate (PET) and polyacrylates are important. Simil arly, the mtural polymers like cotton and silk are important from the point of view of textile app lications. Synthetic polymers like PET, nylon and PP can also be used for manufacturing fibres . Thei r structure depends on many parameters such as temperature, pressure, draw-ratio etc .

Polymers are made up of large chai ns of monomer units with degree of polymerization of one lakhs or above2.:1 . The molecular chai ns arrange themselves in a varie ty of ways forming super­molecular structures. When the chains are arranged in an ordered way, a three dimensional symmetry is maintained and behaves like a crystalline substance. The other parts of the chain are randomly oriented and form amorphous regions. The method of X-ray diffraction (XRD) is, therefore, very well suited for the study of such partiall y crystalline material s~ .

This paper reports XRD studies of PET in the form of film and fibres. The percentage crystallinity and other parameters have been calculated .

2 Experimental Details

2.1 Materials

The PET films ( 12 11m thickness), provided by Mis Garware Polyesters Ltd, India, were cleaned with acetone in ultrason ic bath for I min and dri ed in air. All chemicals used were of AR Grade.

The XRD patterns were recorded us ing Philips X-ray Generator PW 1729 and automat ic X-ray

diffractometer model PW 1710 unit. Copper Ka I line from a sealed tube hav ing copper anode served as a source of radiation. The specimen sample in the form of film was fixed on an aluminium holder. The ho lder was then mounted on the rotating stage of

362 INDIAN J PURE & APPL PHYS, VOL 40, MAY 2002

the diffractometer. Nickel-filtered copper radiation was incident on the sample, which was scanned at I "I min in reflection mode over a range of 28 from 5 to 500.

water in ~

probes

al. shield

magnets al. spacer

r f (ll56MHz)

top plate

glass chamber

s.s. plate

I L-.. ..... ilr---II- monomer/gas " inlet

1IH+-III-----iI- teflon

l,~ater

wnter out

base plate

vacuum

Fig. I - Magnetron enhanced plasma-processing unit

The plasma-processing chamber consisted of a bell jar of diameter 30 cm and height 30 cm. The top and the base plates have various ports. The magnetron was mounted on the base plate. The inlets for the monomer and gas were provided on the base plate. The gas inlet was externally connected to the mass flow controller (unit model URS-IOO). The monomer inlet was connected to precision needle valve, which was first calibrated by mass flow controller. The top plate consists of many ports on which Pirani vacuum gauge was fitted and is depicted in Fig. I

The two stainless steel parallel plates, inside the chamber, were capacitatively coupled with if source. The frequency of power source was 13.56 MHz and can deliver a power of IOOW. Plasma could be created at a low pressure with any

gaslmonomer vapours. Usually air or nitrogen gas was used. The time of treatment was varied from 30s to 30 min. Polystyrene (PS) was deposited on PET substrate film in the plasma created with styrene monomer in the chamber. The working pressure was adjusted to 0.1 mbar. The thickness of film was controlled by thickness monitor.

The PET fibres of commercial variety, 76 denier supplied by Mis Nirlon Ltd, were used. These fibres were treated with swelling agents using dichloromethane (DCM), dimethyleformamide (DMF) and perchloroethane (PCE). The treatment was carried out in a conical flask maintaining the temperature at 40 DC for DCM and 100 DC for other two solvents. After the treatment, the samples were taken out, rinsed with water and dried in vacuum desiccator for 4 hr.

3 Results and Discussion

Typical XRD patterns of some crystalline and amorphous polymers are shown in Fig. 2. It is interesting to note that, as the crystalline structure builds up the diffraction peaks due to crystalline region approach the ideal form of a single crystal. Unfortunately, large single crystals of polymers cannot be grown, as ordering of all the chains is very difficult. However, electron diffraction from very small micro-crystals does yield a pattern resembling that for a single crystal. Typical patterns are shown in Fig. 2. Fig. 3 shows typical pattern for natural fibres cotton and silk. All these patterns show that, the materials are not perfectly crystalline and are oriented to some extent along the fibre axis. The crystal structure for cotton is monoclinic whereas, silk is orthorhombic.

3.1 Percentage Crystallinity and Crystallite Sites

From the radial scans of intensity versus 28, the lateral order or the crystallinity index was determined using Manjunath 's formulaS.

According to the method suggested by Manjunath et al. S. for any polymer, the resolution of peak R is given by:

R = ml + 2m2 +m} + .. .... +mn _ 1

hl+hz+ ···· ·····+hll

BHAT & DESHMUKH : X-RAY CRYSTALLOGRAPHIC STUDY OF POLYMERIC MATERIALS 363

(a)

Fi g. 2 - T ypical XRD patterns of (a) amorphous polymer: (b) part ially crysta lline polymer; (c) o ri ented po lymer

w here, 111 1, 1112, . . . . are he ights of minimum between two peaks, hi, h2, •••. are he ights of peaks from base line.

A typical diffraction curve illu strating the method is shown in Fi g. 4 . Then ( I-R) gives the late ra l order or the index o f c rysta llinity. The c rystallite size was determined by Scherre r's formula4

. For thi s purpose, the peaks were reso lved assuming the Gauss ian profile.

The ha lf-width of the re fl ecti on was measured and the c rysta llite size was calculated using the re lation :

D = KJ.. ".k.! f3 cos e

where f)".u is the crys ta llite s ize in a direc ti on perpendicu la r to the plane (h , k , I ) corresponding to the

measured re flecti on; A the wave length o f radiati on used ( I .S42A) ; K the constant (= 0.9 for set-up of the authors); ~ the half max imum wid th in radians; e is the Bragg ' s angle. Two XRD patte rns were recorded for each samp le and mean val ues are reported .

XRD of PET - PET is made up o f fully extended planer chains of condensed este r of terephthalic ac id and e thylene g lycol. The repeat unit formu la is gi ven be low:

o 0

11 -0-11

~~C ~ ~ C--o-~

n

The chain-re peat di stance is 10.7 A. No specia l forces of attrac tion ex ist between the chain molecules except the Van der W aa l's forces . It is composed of some amorphous and some crysta lline phase depending on the conditi ons of manufacturing . The c rys tal s within the fibres are hi ghl y oriented a long the fibre ax is whe reas the fi lms a re bi -axia ll y ori ented . The non-crysta lline regions in the fibres and the films are responsib le fo r some unique properties like d yin g and hi gh stretchability. The crysta l struc ture of PET is reported to be triclini c and has one reReat unit in the unit ceW with a = 4 .56 A, b = 5 .9 A and c (fibre

ax is = 10.75 A), a =98 .5°, ~ = 11 8° and y = 112°.

A typica l XRD pattern of rando mi zed PET is shown in Fig. 5(a). It consists o f th ree peaks at

17 .5° (010), 23.0° ( 11 0) and 25.6° ( 100). The peak

at 23.0° usua lly has mos t o f the contributi on from the amorphous region and is, the refore, broad. The

364 INDIAN J PURE & APPL PHYS, VOL 40, MAY 2002

other two peaks change thei r relative intensities depending on the orientation of the material.

3S 30 25 20 15

28 DEGREES

Fig. 3 - (a) Typical XRD of cotton cellu lose

If) t-Z ::>

~ 0:: t-m cr: <t: z >­t-V) Z W t-t;

5 10 15 20 25 30 35 DIFFRACTOMETER ANGLE 2 e IN

DEGREES

Fig. 3 - (b) Typical XRD of mulberry si lk

10

XRD patterns from the fibre samples studied in the investigations of the authors are shown in Fig. 5. When the fibres were treated with DMF some peaks become sharp and re lative intensity has changed. The percentage crystallinity and crystallite sizes

were calcu lated as outlined above. It was seen that the percentage crystallinity increases with time of treatment as well as the temperature of treatment from 17% to 50% (Table I ). The treatment was a lso carried out with DCM for diffe rent temperatures and it was noted that, the percentage crystallinity increased from 17% to 35 %. Similarly, in case of PCE it was noted that, the change in crystallinity was from 17% to 47%.

I/t ~ -Z :J

>-a: ~ a: t--til Q: ~

Z

>-t-

VI Z W f-z;

10 IS 20 2S 30

DIFFRACTOMETER ANGLE 29°

Fig. 4 - A typical XRD curve showing the method of calculat ion of resolution factor

Simi larly, crystallite sizes were calcu lated from the half peak width of all the three reflections noticed. It was found that crysta llite s ize a lso increases from 36 to 47 A for (0 I 0) plane when treatment was carried out in different so lvents.

BHAT & DESHMUKH: X-RAY CRYSTALLOGRAPHIC STUDY OF POLYMERIC MATERIALS 365

Vl >-Z :J

6: « 0:: >-ro 0:: « z >­>-Vi Z llJ >­Z

12

(a) CONTROL (b) 100°C, 30 MIN, DMF TREATED (e) 140°C, 30 MIN, DMF TREATED

(100)

16 20 24 28 32 28, DEGREES

a 36

Fig. 5 - X-ray diffractogram of control and treated PET fibres

Table I - Effect of pre-treatment on the density of PET filament s pre-treated with DMF at various temperatures and durati on of time

Treatment Treatment Densi ty Crystallinity temperature time in min g1cm~ index X

°C

Contro l 1.354 0.1691

100 5 1.375 0.3483

100 15 1.375 0 .3483

100 30 1.380 0 .3925

100 60 1.383 OAI76

120 5 1.38R 0.4590

120 IS 1.390 0 .-1756

120 30 1.390 0 .4756

120 60 1.391 (lA842

140 5 1.388 0.4590

140 15 1.389 0.4678

140 30 1.391 0 .-1842

140 60 1.394 05094

.J... (c)

(b)

~ (0)

-L I 1

5 10 15 20 25 30 35 40 45 50

28 DEGREES

Fi g. 6 - XRD pattern of (a) polystyrene deposited on PET substrate in pi a rna; (b) PET fi lm treated in air plasma for 30 min ; (c) control PET film

Fig. 6(c) ill ustrates the XRD of PET control film . It can be seen that , ou t of three peaks onl y

peak at 25.6° coul d be seen prominentl y whereas ,

peak at 17 .5° has very low intensity . Thi s shows that , ( 100) c rystal planes are hi ghl y orie nted along the surface of the film as revea led by th is XRD pattern . Fig. 6(b) shows the changes when the film was subjected to air plasma treatment. It was seen thnt, there was no apprec iable change in the diffraction profi le . However- the re lative intensity o f peak at 17.5° has sl ightly increa. ed. Calcu lations show that the percentage crystall ini ty has increased

366 INDIAN J PURE & APPL PHYS, VOL 40, MA Y 2002

from 69.3 to 72 .5 Fig. 6(a) shows XRD pattern of PET f ilm treated with styrene in plasma chamber. It can be seen that , the intensi ty of the main peak ( 100) has tremendous ly decreased showing that PET film is coated uniformly with po lystyrene. The

r lati ve inte ns ity in the region of 23° has somewhat decreased and the calcul at ions for the percentage crys tallinity show the change from 69 .3 to 64.6%.

References

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York), 1999.

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3 Govari kar V R et a i, PolYlll er sciell re (New Age Internat ional (P) Ltd , New Delhi ), 1986.

4 Alexander L E, X-ray diffractioll lIIethods ill polVllla sciellce (Wiley Interseienee, New York) , 1969.

5 Manjunath B R et a i, J Appl Polrlll Sci, 17 ( 1973 ) 1091 .

6 Murthy N S, Correale S T & Minor H, Ma crolllolecliles, 24 (199 1) 11 85.