Ladder-type copolymers—I. Investigation of the molecular structure

Eur. Polym. J. Vol. 24, No. 2, pp. 177-181, 1988

0014-3057/88 $3.00+0.00 Copyright © 1988 Pergamon Journals Ltd

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LADDER-TYPE COPOLYMERS--I. INVESTIGATION OF THE M O L E C U L A R S T R U C T U R E STANISLAW RABIEJ and

ANDRZEJ WLOCHOWICZ

Textile I n s t i t u t e , T e c h n i c a l U n i v e r s i t y o f Lode', Bielsko-Biata, F i n d e r a 32, 4 3 - 3 0 0 B i e l s k o - B i a l a P o l a n d

(Received 10 December 1986; in revised form 14 April 1987) A b s t r a c t - - T h e m o l e c u l a r s t r u c t u r e o f l a d d e r - t y p e s t y r e n e - m u l t i m e t h a c r y l a t e c o p o l y m e r s h a s b e e n inv e s t i g a t e d b y G P C a n d i.r. G P C h a s b e e n u s e d f o r d e t e r m i n a t i o n o f m o l e c u l a r w e i g h t d i s t r i b u t i o n s a n d w e i g h t - a v e r a g e a n d n u m b e r - a v e r a g e m o l e c u l a r weights. C o m p o s i t i o n s o f c o p o l y m e r s h a v e b e e n determ i n e d b y i.r. s p e c t r o s c o p y .

INTRODUCTION

Styrene-multimethacrylate ladder-type copolymers are synthesized by an original method involving matrices [1]. A matrix is a compound which fixes monomers and causes a particular three-dimensional arrangement. It has an important effect on the constitution of the product, which is formed as a result of polymerization of these monomers. Macromolecules of ladder-type copolymers are built of alternating multimethacrylate blocks with fixed length and polystyrene blocks with variable uncontrolled lengths. The term "ladder-type copolymers" is connected with the constitution of the multimethacrylate blocks, for which the system of bonds and atoms resembles a ladder. Considering their constitution, the blocks are more rigid than the styrene blocks. The molecular weight of monomeric units of multimethacrylate matrix (M = 188) is almost twice that of styrene. The present paper is the first of a series concerned with investigations of the supermolecular structure of solid ladder-type copolymers. The investigations were carried out from the point of view of the relationship between structure and chemical constitution of copolymers. The results of measurements of component weight fractions, molecular weights and their distributions are now presented.

with the matrix, were united into blocks, with lengths dependent on the matrix length. The copolymer was precipitated with hot methanol and dried under reduced pressure at 90°C. The scheme of copolymerization on the matrix is shown in Fig. 2. Using the radioisotope-labelled initiator, Polowifiski [4] found that the copolymers have a linear constitution and that branching is insignificant. The copolymers used in this study were different in respect of sizes of multimethacrylate blocks and weight fractions of multimethacrylate and polystyrene. The differences were achieved by selection of the weight ratio of components in the initial mixture and by use of selected fractions of p-cresolformaldehyde resins with required molecular weights. The investigated copolymers are listed in Table 1. Homopolymers, i.e. polystyrene and multimethacrylate homopolymer (polymerized multimethacrylate) were also investigated. DETERMINATION OF C O P O L Y M E R C O M P O S I T I O N S

Compositions of ladder-type copolymers were determined by i.r. spectroscopy. The spectra were recorded with the aid of Beckman IR 4220 and Table I. Molecular structure of ladder-type copolymers. Results of measurements

T H E SYNTHESIS O F LADDER-TYPE C O P O L Y M E R S

Styrene-multimethacrylate ladder-type copolymers were synthesized by means of the method developed by Polowifiski [2-4]. In the frst stage, fractions of p-cresol-formaldehyde resin were obtained by polycondensation. The selected fractions (matrixes) of known composition and molecular weight were esterified with methacryloyl chloride in the presence of triethylamine. Multimethacrylates (Fig. 1) were obtained as a product of the reaction. The copolymerization of multimethacrylate and styrene was carried out in benzene at 90°C using AIBN as an initiator. The concentration of comonomers was 71.4 g/l and the concentration of initiator was 1.42 g/l; the reaction time was 9.5 hr. Methacrylate units, connected

Sample

%MM

Number of monomers in MM blocks

Mw

h4,

D

74 70 71 63 62 65 63 53 41 31 44

4 4 4 4 5 6 8 4 4 10 8

36 800 -31 400 29 100 --21 700 29 700 16 600 23 600 --

10 100 -10 400 9 400 --l0 600 10 400 8 200 I 1 700 --

3,64 -3,02 3,t0 --2,05 2,86 2,02 2,02 --

A

B c D, 02 D3 D4 E F G H

% M M - - w e i g h t fraction of multimethacrylate, h,7"w--weight-average molecular weight. h~n--number-average molecular weight.

o = ~w/~o. 177

178

STANISLAW RABIEJ and ANDRZF_J WLOCHOWICZ

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y

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I

I

I

I

I

I

l

I

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I

40

I

I

I

1

I

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Fig. 3a. i.r. Spectrum of polystyrene. Band 3030 era-J used in quantitative analysis is indicated.

80

60

40

20

I I I I L

I I I I

II

40

lO

20

k. 10 -z [ c m -1"1

Fig. 3b. i.r. Spectrum of polymerized multimethacrylate. Band 1735 cm -~ used in quantitative analysis is indicated.

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I

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r

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k . 1 0 - 2 r cm-1"l

Fig. 3c. i.r. Spectrum of ladder-type copolymer-sample F(41% MM).

STANISLAW RABIEJ and ANDRZEJ WLOCHOWICZ

180

Beckman IR 4230 spectrophotometers. For qualitative analysis, the spectra of powdered samples in the form of tablets with KBr (Figs 3a-c) were used. Quantitative analysis was performed using spectra of tetrachloroethylene solutions of investigated substances with concentration of 10 mg/ml. The quantitative analysis is based on LambertBeer law, which gives the dependence of absorbance on concentration of given component. A =log-~=abc

(1)

where A = the absorbance at given wavenumber a = the absorbance constant for given component at given wavelengths b = the path length of radiation in the sample c = the component concentration P = the intensity of transmitted radiation P0 = the intensity of incident radiation From Eqn (1) it follows that the dependence of absorbance (A) on concentration (c) should be linear but this is not always the case. Usually this dependence is linear only for low concentrations [5-6]. The measurements of unknown concentrations were performed on the basis of master curves, plotted as a result of absorbance measurements for seven standards. Solutions of mixtures of polystyrene and multimethacrylate homopolymer of known compositions were used as standards. The spectra of standards and investigated copolymers were recorded using the same measurement cuvette. The solvent spectrum was subtracted by a compensation method. F o r determination of master curves, the following bands were chosen: 1375 c m - l for multimethacrylate (band M in Fig. 3a) and 3030 c m - i for polystyrene (band PS in Fig. 3b). The master curves are shown in Fig. 4. Concentrations of multimethacrylate and polystyrene and weight ratios of these components were found on the basis of absorbance values, which were read off from the spectra. For all the investigated copolymers, corresponding absorbances were situated in the linear parts of the master curves. The results are listed in Table 1. The absolute error of composition determinations was not greater than 2%.

DETERMINATION OF MOLECULAR WEIGHTS OF COPOLYMERS

GPC was applied for the determination of molecular weights of copolymers. The measurements were carried out with a ALC GPC 301 Water Associates liquid chromatograph. The conditions were as follows: solvent, tetrahydrofuran; flow rate of solvent, 1 ml/min; concentration of sample, 2mg/ml; detection, a differential refractometer and a u.v. photometer with 2 = 254 nm. Columns calibrated with styrene standards were used. The master curve for read-out of molecular weights was determined for seven monodisperse styrene standards (Waters Ass.). It was found that ladder-type copolymers are characterized by unimodal molecular weight distributions for polymeric fractions, which are in the eluate up to an elution volume of 125ml (corresponding to a molecular weight of 103). Widths of distributions are greater for copolymers with high weight fractions of multimethacrylate. Some typical chromatograms are _.~resented in Figs 5 and 6. Integral molecular weight istribution curves representing the dependence of cumulative fraction masses on the molecular weight were constructed on the basis of chromatograms and the master curve. Examples are presented in Fig. 7. Weight-averge (h,Stw) and number-average (h~'n) molecular weights as well as indexes of heterogeneity (D = h~,/h~n) are given in Table 1. Apart from the main maximum, which corresponds to the polymeric

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Fig. 4. The master curves used for determination of copolymer compositions: A--adsorbanee; C-----concentration

in solution; M--the master curve for multimethacrylate absorbance for 1735 cm -~ band; P~-the master curve for polystyrene absorbance for 3030cm -~ band.

I 90

I 100

I 110

I 120

I 130

I 140

V e rn

I

I

I

I

I

6

5

4

3

2

tog M Fig. 5. GPC chromatograms of ladder-type copolymers: curves I a, 1b---sample F (41% M M); curves 2a, 2b--sample A (74% MM). : Indications of u.v. differential refractometer; - - - : indications of u.v. photometer.

181

Ladder-type copolymers--I Table 2. The height ratios for peaks recorded with the u.v. detectorand the differentialrefractometer S = Hw/H,

Sample

I 80

J 90

I I 100 110

F E D4 C A

I [ I 120 130 140

Ve [rnt.]

i 5

I 4 Log M

I 5

I 2

S

Weight fraction of MM%

Main peak

Marginal peak

41 53 63 71 74

1,89 2,13 2,22 2,37 2,5

4,67 7,67 6,52 9,0 4,4

PS

1,5

PS--polystyrene standard.

Fig. 6. GPC chromatogram of sample D4 (63% MM). material, a small peak is visible on chromatograms at elution volume of about 135 ml. Molecular weights of this fraction are between 100 and 500. Taking into account the ratio of the area under the additional peak to the total area of the chromatogram, in the co-ordinate system nD(M ) the weight concentration of low-molecular weight fraction was estimated as not exceeding 0.1% for all investigated samples. The height ratios for peaks recorded with the u.v. detector and the differential refractometer (S = H,~/H~) were calculated. The calculations were performed both for the main maximum and the peak corresponding to low-molecular weight fraction. Table 2 compares values obtained for five samples with increasing multimethacrylate concentration. The values for the main maximum indicate that copolymers absorb u.v. radiation to a greater degree than pure polystyrene (PS); furthermore, absorption increases with increasing weight fraction of multimethacrylate. It is evident that multimethacrylate causes greater u.v. absorption. Taking it into account, one should assume that the low-molecular

80

weight fraction consists mainly of short (1-3 units) multimethacrylate blocks and small amounts of styrene, since the ratio is still greater for this fraction. The molecular weight of a three-unit block is 553 whereas the molecular weight of styrene is 104. These values agree with the molecular weight range found for this fraction from chromatograms.

SUMMARY

The investigation shows that ladder-type copolymers obtained by copolymerization on matrices are characterized by unimodal molecular weight distributions with small amounts of low-molecular weight impurities. Average molecular weights are between 2 x 104 and 3 x 104. Heterogeneity of molecular weights is considerable. The index of heterogeneity is greater for samples with high content of multimethacrylate. Investigations of 1 ! types of copolymers, with weight fraction of polystyrene in the range from 26% to 69%, were performed. In this group, there were 4 copolymers which differed in multimethacrylate block sizes in spite of similar composition with about 37% of polystyrene. These blocks contained from 4 to 8 monomeric units.

I REFERENCES

~ 60

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u') 40 20 t

I I tltltl

~'~lJl/lltlld

IO 3

I

104

I IIIIIll

I

105

I lilILIJ

#106

Fig. 7. Integral molecular weight distribution curves: l--sample F (41% MM); 2--sample A (74% MM).

1. H. Kammerer. Makromolek. Chem. 111, 67 (1968). 2. S. Potowifiski. Int. Conf. of Macromolecular Compounds. Budapest (1969). 3. S. Polowifiski. Polimery 17, 409 (1972). 4. S. Po|owifiski. Eur. Polym. J. 14, 563 (1978). 5. N. Alpert, S. Keiser and H. Szymafiski. Theory and Practice of Infrared Spectroscopy. Plenum Press, New York (1970). 6. K. E. Steine. Modern Practices in Infrared Spectroscopy. Beckman Instruments (1970).