NMR investigation of compatibility in blends of poly vinyl chloride and ethylene-vinyl acetate copolymer—II

European Pol)mcr Journal. Vol 13. pp 95 to 98 Pergamon Press 1977 Printed in Greal Britain

NMR INVESTIGATION OF COMPATIBILITY IN BLENDS OF POLY VINYL C H L O R I D E AND ETHYLENE-VINYL ACETATE C O P O L Y M E R - - I I C. ELMQVIST Department of Physical Chemistry, University of Gothenburg. S-402 20 Gothenburg, Sweden

(Received 6 April 1976) Al~traet--Measurements have been made on poly vinyl chloride, ethylene vinyl acetate copolymer (EVA. Levapren 450 M. 45 wt~ VAc) and their mixtures. From wide-line NMR measurements, three different relaxation mechanisms for EVA were found. Spin-lattice relaxation times have been measured in the laboratory frame (T0 in the temperature range 220--450°K. A precipitated sample is shown to contain intermediate phases as well as portions of free polymer components. The same applies to an extruded graft-polymerized sample. By heating this last sample, partial separation of the components occurs. The PVC/EVA (45 wt~o VAc) system can be regarded as a semi-compatible system.

INTRODUCTION

RESULTS AND DISCUSSION

Ethylene-vinyl acetate copolymers (EVA) are the best impact strength modifiers for PVC; the addition of 5-10 wt% EVA containing 45 w t ~ vinyl acetate seems to give the best properties [1]. As a knowledge of the phase relations is essential for understanding the effect of impact strength modifiers, we have previously studied the PVC/EVA (45 w t ~ VAc) system by wideline N M R [2--4] and by differential scanning calorimetry (DSC)[5]. Different opinions regarding the phase relations in this PVC/EVA system have been reported [6-10] and therefore we wish to confirm, by a pulsed N M R method, our earlier result concerning the existence of a mutual partial compatibility in the PVC/EVA (45 wt~o VAc) system.

When comparing our samples without stabilizing and lubricating additives with the pure polymer components (Fig. I), a PVC sample without these additives was used, this being the same PVC powder used for preparing our samples. For the graft-polymerized and extruded sample, the comparison was made with an extruded PVC rod containing the additives (Fig. 4). The position and the magnitude of the minimum in the logarithm Tlvs reciprocal temperature plots are the same for the two PVC samples. However, at low temperatures the relaxation times of the powder and of the rod reach values of about 1 sec and 650 msec respectively. At the minimum in a T~ plot, for a simplified behaviour of a two-spin system, the resonance frequency (COo) times the correlation time (re) is a constant [11]:

EXPERIMENTAL

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The polymers used were suspension-polymerized PVC (M,. = 83,000) and EVA containing 45 wt~o vinyl acetate (Levapren 450 M, Bayer AG, W. Germany). By evaporation of the solvent from an EVA solution containing suspended PVC powder, samples with the components completely separated in two phases were achieved. In order to obtain the best degree of mixing, samples were prepared by precipitation. Kema Nord AB, Sundsvall, Sweden manufactured graft-polymerized samples (rods) by suspension-polymerization of vinylchloride in the presence of EVA followed by extrusion. These last samples contain totally 5 wtVo stabilizing and lubricating additives. A more detailed account of the preparation of the samples is published elsewhere [3]. The EVA polymer was converted to ethylene-vinyl alcohol copolymer and then reacetylated with trifluoroacetic acid and its anhydride to give ethylene-vinyl trifluoroacetate copolymer, which was used when identifying the different relaxation processes in EVA. A Brucker pulsed NMR spectrometer B-KR 322s 4-62 MHz was employed. The working frequency was chosen to be as low as 19.4 MHz because it was desired to observe the PVC glass transition at a moderate temperature. The spin-lattice relaxation time (Tt) was obtained through a 180°-r-90 ° pulse sequence, where the time r varied widely in order to register the whole signal. The wide-line NMR measurements were performed with a Varian 4250 wideline spectrometer stabilized at 16 MHz.

In these measurements the corresponding correlation frequency is 31.5 MHz. The PVC glass transition can to a good approximation be considered to reflect a relaxation mechanism with this frequency occurring with greatest efficiency at 415°K. The shallow T~ minimum of the EVA polymer is ascribable to a span in the distribution of correlation times. The effect on T1 is also to raise its minimum value since all the motions causing relaxation are not equally effective at a given temperature. In the wideline N M R line width vs temperature curve for EVA (Fig. 2), there are three different relaxations: (I) the glass transition (around 255°K) from the motion of isolated - - - C H 2 - - - C H A c - - C H 2 - groups[12] (the participation of the acetate groups is evident from Fig. 2); (2) anisotropic rotations in short ethylene sequences[13] (around 220°K); and (3) decreasing amplitudes of torsional motion of the chains (around 140°K). The threefold methyl rotation is free at least down to 80°K. The methyl groups have been distinguished by performing F 19 and H 1 measurements on EVA with the acetate groups replaced with trifluoroacetate. For the different relaxation processes. 95

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Fig. 1. The reciprocal temperature dependence of the spin-lattice relaxation time (T~) in, • pure PVC; • pure EVA; O PVC/EVA (45 wt% VAc) blend with 25 wt% EVA. components separated: x PVC/EVA (45 wt% VAc) blend with 25 wt% EVA. precipitated. 20

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Fig. 2. The temperature dependence of the wide-line NMR line width (peak-to-peak distance) in EVA and in ethylenevinyl triltuoroacetate copolymer (EVA-F), • pure EVA; • H l resonance in EVA-F; O F 19 resonance in EVA-F. activation energies have been calculated by the Hendrickson and Bray method [14] (Table 1). The apparent activation energy [15] of EVA calculated from the Tl measurements is approximately 20 kJ/mol; from a comparison of the different EVA activation energies, it is evident that the T~ minimum reflects several relaxation mechanisms. Of course there is uncertainty

in the meaning of the apparent activation energy when several relaxations are involved but for comparative purposes the value is useful. Two spin-lattice relaxation times can be extracted from spectra of samples containing 25 wt% EVA and with the PVC and the EVA polymers separated in two phases (Fig. 3). In order to distinguish two relaxations, the quotient between the long and the short relaxation times must however be at least a factor 2 [16]. Forty-three per cent of the total number of protons are attached to the EVA chains in a 25 wt~ sample and thus, in Fig. 3, the experimentally found 47~o indicates a complete separation of components. Similar values are found for the whole temperature range. The T1 values coincide closely with those of the pure polymers except in the PVC minimum whose value is raised from 24 to 33msec (Fig. 1). In the precipitated PVC/EVA (45 wt% VAc) blend with 25 wt% EVA, there seems to exist several phases with different degrees of mixing as well as certain amounts of pure polymers. The minimum at 383°K (Fig. 1) appears rather shallow and would probably be even shallower if an extraction of the pure EVA relaxation were possible for temperatures above that of the minimum. The temperature at the minimum is shifted to lower values compared with that of PVC and the shift suggests a solution softer than the brittle PVC. The appearance of one minimum suggests high degree of homogeneity but the high value of the Tl minimum (95 msec) compared with those of the individual polymers, 24 and 45 msec for PVC and EVA

Table 1. Activation energies calculated according to Hendrickson and Bray [14] from wide-line NMR measurements on EVA polymer Relaxation process Glass transition Glass transition seen by the methyl group Anisotropic rotation in short ethylene sequences (220°K) Torsional motion in the main chain (140°K)

AE, kJ/mol 80 65 8.8 3.8

NMR investigation of compatibility in blends of pol~ vinyl chloridc

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Fig. 3. Experimental magnetization curve and evaluation diagram of PVC/EVA (45 wt°,, VAc) blend with 25 wt°o EVA, components separated, at 417K. The ordinate of the evaluation diagram has been taken as the difference in arbitrary units between the equilibrium magnetization, indicated at the upper right and left of the curve, and magnetization at time z.

respectively, suggests a heterogeneous blend. A continuous phase separation starting above 395:K is observed when measuring. This is evident from the intensity values of the long relaxation time which are, starting at the highest temperature measured, 60. 65, 70, 75 and 80 proton ° o. For the unseparated sample the amount of PVC in the mixed part of the blend has been estimated to 50 wt°/o. The sample contains totally 75 wt°.~, PVC. The lineshape of the curve from the minimum to about 290~K indicates the existence of intermediate phases and, at even lower temperatures, a steady value t)f 230msec is reached. As a whole the T~ values to the right of the minimum form a PVC shaped curve of markedly low relaxation values, showing solubility of EVA chains in the PVC matrix In the wide-line N M R spectrum of this sample, an appreciably narrowed broad line appears. interpreted as due to softened PVC [3]. It has also been possible to detect 7"1 values of the free component polymers, however, the failure to detect the PVC relaxation below 350°K is probably explained

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by the high T1 value combined with the amount of free PVC present From DSC measurements it was concluded that several phases with different degrees of mixing of the polymer chains as well as free PVC exist in this precipitated sample [5]. The grafted and extruded sample containing 7.5 Wt°o EVA (a commercial product) shows a compatibility structure much like that of the precipitated blend (Fig. 4). The minimum temperature shift towards a softer structure is 18 ~. about half the value of the 25 wt~o EVA precipitated sample. For temperatures to the right of the minimum, the relaxation time is lower than for PVC. The low molecular additives might cause this lowering but their influence is believed to decrease the relaxation values from 1 sec to 650 msec as previously mentioned when PVC with and without additives were considered, while the further decrease is presumed to be due to EVA dissolved in PVC. The precipitated sample of PVC + EVA only shows that EVA can cause a similar decrease. Relaxation times corresponding to the

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Fig. 4. The reciprocal temperature dependence of the spin-lattice relaxation time in, • extruded PVC: × extruded PVC/EVA (45 wt~o VAc) blend with 7.5 wt~o EVA where the vinylchloride is suspensionpolymerized in the presence of EVA; O the same sample heated at 443K for 40 min. The EVA curve is redrawn from Fig. 1.

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fore that the same applies to the PVC/EVA minimum. Therefore the change observed in the PVC/EVA curve on heating cannot be explained by transfer of additives from the mixed phases or from the EVA phase to the pure PVC. although the intensity percentage would allow this possibility. In addition, the experimental magnetization curves of the PVC rod are exponential over the whole temperature range. showing a true solubility in the rod between PVC and the additives. The decrease from 50 to 25~o in the intensity of the long relaxation, due to heating, is interpreted as being due to a separation of PVC from the mixed part of the blend. A quantitative analysis of the EVA in this sample is not possible owing to its low total content. The wide-line technique is in this connection more successful. Compared with the precipitated sample, a surprisingly weak tendency to separate when measuring at the highest temperatures was observed and the separation caused was negligible compared with that produced by the heat treatment. This suggests a difference in the physically (precipitated) and the chemically (graft-polymerized) mixed samples.

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Acknowledgements--The author thanks Dr. S. E. Svanson and Fil. kand. L. G. Svensson for valuable discussions. Thanks are also due to Prof. S. Fors~n. Department of Physical Chemistry, LTH. for making the pulsed NMR spectrometer available.

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Fig. 5. Experimental magnetization curves and evaluation diagrams of (a) extruded PVC/EVA (45 wt~o VAc) blend with 7.5 wt~o EVA where the vinylchloride is suspensionpolymerized in the presence of EVA, and of (b) the same sample heated at 443°K for 40min. The measuring temperature for both samples is 405°K. The time T in (a) is obtained by multiplying the time scale by the factor 2 or 4. pure polymers are also observed in Fig. 4 although not resolved for all temperatures. For clarity, the curve for pure EVA is redrawn from Fig. 1. Heating the graft-polymerized sample at 443:K for 40min in nitrogen is believed to separate the component polymers[3,4]. For low temperatures the relaxation time for the mixed portion of the sample is raised to 650 msec, the value characteristic of the extruded PVC rod. For high temperatures an increase is also observed, although not corresponding to complete phase separation. From the intensity relations obtained when evaluating the experimental magnetization curves measured at 405°K (Fig. 5), it is concluded that the long relaxation is decreased from 50 to 25~o of the total proton content, when the sample is heated. As earlier pointed out, the PVC m i n i m u m is unaffected by the additives and it is probable there-

REFERENCES

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