Multiphase Polymers - American Chemical Society

30 Effect of Branch Length of Neoprene-g-

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Poly(tetrahydrofuran) Copolymers on Properties J. LEHMANN and P. DREYFUSS 1

Institute of Polymer Science, University of Akron, Akron, O H 44325

Neoprene-g-poly(tetrahydrofuran) copolymers with branches of different length were synthesized. Solubility, NMR, and GPC indicated that pure graft copolymer was produced in all cases. The copolymers exhibited one T which was between the T s of the homopolymers and a crystalline melting temperature slightly lower than the T of polytetrahydrofuran. Polarizing optical microscopy on thin films revealed spherulitic domains in a nonspherulitic matrix. Tensile strength tests of uncured specimens indicated that breaking-elongation decreases and ultimate tensile strength increases as the length of the polytetrahydrofuran branches increases. Permanent set increased with branch length and was thermally reversible. Adhesive characteristics in standard neoprene recipes were very similar to those of neoprene or polytetrahydrofuran with plastic or metal adheronds. g

g

m

" \ T 7 h e n certain allyl halides react with a suitable inorganic salt i n the * * presence of a heterocyclic monomer, polymerization ensues ( I ) . If the halide is a polymeric halide, graft copolymer results. The objectives of the present work are: ( 1 ) to synthesize and characterize such graft copolymers and (2) to test some of their mechanical properties and relate them to the content of branch which has been grafted from the polymer. Current address: Electrochemical Industries ( Frutarom ), P.O. Box 1929, Haifa 31000, Israel. 1

0-8412-0457-8/79/33-176-587$05.00/0 © 1979 American Chemical Society Cooper and Estes; Multiphase Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

588

MULTIPHASE

POLYMERS

As the polymeric halide for our initial investigation we have chosen polychloroprene (Neoprene), which has the following structure: -f CH —C=CH—CH HCH —CCl+f CH —CH 2

2

I

2

2

I

Cl

!

CH=CH

2

j-

CC1=CH

2

~ 98% ~ 1.5% - 0.5% The tertiary allylic chlorides labeled above with an asterisk are the graft ing sites ( J ) . Tetrahydrofuran ( T H F ) was chosen as the heterocyclic monomer because its chemistry is so well known that it is the model heterocycle of choice i n oxonium-ion polymerizations ( 2 ) . This chapter describes the synthesis of the selected graft copolymers and some of their characterization by solubility, N M R , IR, G P C , D T A , optical microscopy, stress-strain properties, and work of adhesion. Corre lations with polytetrahydrofuran ( P T H F ) content and branch length are reported. These correlations are based on calculated numbers derived from gravimetrically determined conversions and the number of active halogens on the backbone assuming 100% reactivity of the halogens. Studies with model, small organic halides suggest that this is a reasonable first approximation (3,4). Attempts to characterize the branches experi mentally have been unsuccessful so far. Experimental Materials. T H F monomer was refluxed under nitrogen first over lithium aluminum hydride for 48 hr, then over sodium-potassium alloy. The distilled monomer was used within 24-48 hr. Methylene chloride ( C H C 1 ) was refluxed over calcium hydride overnight under nitrogen and then distilled under nitrogen. Silver hexafluorophosphate ( A g P F ) ( A l f a Inorganics or Ozark Mahoning) was used as received. Triphenylphosphine was recrystallized from ether and dried in a vacuum oven at room temperature. Polychloroprene ( Neoprene W or A C Soft, Ε . I. duPont de Nemours and C o . ) was dissolved i n toluene, precipitated i n methanol, and dried i n a vacuum oven at room temperature. This procedure was repeated three times. T h e polymer was used immediately after the last precipitation. Neoprene W and Neoprene A C Soft have M s of 180,000 and 230,000, respectively, and allylic chloride contents of 1.45 and 0.77 m o l % , respec tively. The M„s were determined by osmometry, and m o l % allylic chlo ride are based on an I R measurement of the 1,2-addition-product band of polychloroprene at 921 c m " ( 5 ) . P T H F used for D T A and stressstrain measurements had ΤνΓ = 95,000. The compounding ingredients for preparation of adhesion test speci mens, magnesia ( M g O ) , zinc oxide, Zalba Special ( hindered phenol antioxidant from Ε . I. duPont de Nemours and C o . ) , and Bakelite-brand t-butylphenolic resin CK-1634 (Union Carbide Corp.) were used as 2

2

6

n

1

η

Cooper and Estes; Multiphase Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1979.

30.

L E H M A N N A N D DREYFUSS

589

Effect of Branch Length

received. Aluminum, chrome, and brass substrates were prepared as previously described (6). Poly-2,6-dimethyl-l,4-phenylene oxide ( P P O , General Electric C o . ) sheets were molded between preheated Ferrotype plates backed by stainless steel plates at 290°C i n a Pasadena Press using 30,000 lb on a 5 in. ram. A cycle with 1.5 min preheat and 2.2 min at full pressure followed by 5 min i n a cold press at the same pressure was most suitable. Polyethylene terephthalate ( Mylar, Ε . I. duPont de Nemours and C o . ) was secured to an aluminum plate using a polyester resin (Adhesive 46971, Ε . I. duPont de Nemours and Co.) and pressing for 1 hr at about 125 °C and 20,000 l b per 5-in. ram force. Polymerizations. Graft copolymerizations were carried out i n bot tles or round-bottom flasks under a dry nitrogen atmosphere i n a dry box or under a dry nitrogen blanket. The backbone was first dissolved i n the monomer by letting a mixture of purified monomer, polymer, and i n some cases C H C 1 stand under nitrogen for 24-48 hr with occasional shaking. A g P F solution i n C H C 1 was added with shaking. T h e reactions were terminated by addition of either concentrated ammonium hydroxide or triphenylphosphine under nitrogen. W h e n termination was completed, an antioxidant ( a 1 % solution of l,2-dihydro-2,2,4-trimethylquinoIine i n T H F or C H C 1 ) was added. The copolymers were then isolated by evaporating volatiles and drying in vacuum. The copolymerization details are given i n Table I and the resulting graft copolymer compositions are shown i n Table II. P T H F for D T A and stress-strain measurements was similarly prepared except that allyl chloride was used instead of neoprene. Characterization. Extractions were carried out by stirring the poly mers with several changes of the desired solvent until no more material would dissolve. I R spectra were obtained on a Perkin-Elmer 521 Grating Infrared Spectrometer. Polymer films were cast on a N a C l crystal from a suitable solvent ( usually C C U ). Compositions were determined from Ή N M R spectra using C C 1 solutions and a Varian T-60 N M R spectrometer. Gel-permeation chromatograms of dilute polymer solutions i n T H F at 37 °C were obtained on a Waters Associates Ana-Prep Chromatogram. Details of the G P C analysis have been described (7,8). Number-average molecular weights were measured using a Mechrolab 500 Series Membrane Osmometer and toluene at 38°C. D T A meas urements were made using a duPont 900 Instrument with a D S C cell and a heating rate of 20°C/min. Stress-strain measurements and adhesion tests were made with a table-model Instron Model T N N . Microdumbbells for tensile tests were cut from films cast from C H C 1 . Optical microscopy studies were made using a Leitz Orthoplan microscope with and without crossed polaroids. Samples for 180° peel tests were prepared according to standard neoprene recipes (9). Neoprene A C or a graft copolymer prepared from it was mixed on an open mill at room temperature for 20-25 min with the following compounding ingredients i n succession: 2

2

e

2

2

2

2

4

2

Material Neoprene A C (or neoprene-g-PTHF) Magnesia Zalba special Zinc oxide

2

Parts 100 4 2 5


590

MULTIPHASE

POLYMERS

Table I.

Graft

Neoprene No.

Active CI (mmol)

(g)

1 2 3 4 5 6

Monomer (mL)

b

PTHF

from Neoprene W

0.80 1.53 0.79 1.56 0.79 1.56

0.18 0.25 0.13 0.26 0.13 0.26

22 20 20 20 20 20

PTHF from Neoprene AC Soft 19.38 20.10

7 8

1.7 1.7

150 150

° All polymerizations were run at room temperature. By IR analysis. Terminated with N H 4 O H . Other polymerizations were terminated with 1-2 parts Ph3p based on the AgPFe. This is equivalent to 2-3 parts PI13P based on active CI. b

0

The milled stock (111 parts) was dispersed i n C H C 1 and a solution of ί-butylphenolic resin (40 parts) i n C H C 1 was stirred i n . Additional C H C 1 was added so that the final slurry had 20% solids. The solvent was then evaporated and a thin film was produced. This film was pressed against the prepared substrate and covered with a sheet of finely-woven cotton cloth. The final sandwich was first pressed i n a mold for 30 min at room temperature and 25,000 lb on a 5 in. ram and then cured for 40 min at 150°C and 30,000 lb on a 5 in. ram. The thickness of the elastomer layer i n the resulting cloth-elastomer-substrate sandwich was ~ 0.4 mm. Peeling experiments were carried out on strips of cloth-backed elastomer layer after trimming them to a uniform width on the substrate of 2 cm. 2

2

2

2

2

2

Table II.

Graft Copolymer Compositions Composition'

No. 1 2 3 4 5 6 7 8

Backbone 78 68 40 14 13 10 86 31

(%)

Branch 22 32 60 86 87 90 14 69

(%)

Branch * M„ Χ ΙΟ-

3

1.75 2.68 8.75 36.4 42.9 52.2 1.10 25.7

By m NMR. Calculated from conversion assuming all the allylic chlorines of the backbone have reacted with AçPFe to give active sites for the graft copolymerization. e

b


30.

L E H M A N N

A N D

Copolymerization Details

CHgClg (mL)

0

AgPF (mmol) 6

PTHF 0.2 0.4 0.2 0.4 0.2 0.3

10 11 11

PTHF 80 80

591


D R E Y F u s s

4.8 3.0

Pzn. Time (hr) from Neoprene W 1.2* 4.0 6.7 46.5 28 78

17° 2.5 5.5 3.2° 30° 69.5 from Neoprene AC Soft

2.5 30

3.25 48

* The conversion was low for the time polymerized because of an impurity in the neoprene. The polymer is included in the table because it was used for some of the characterizations described below.

The cloth-backed elastomer layer was peeled off a short distance, bent back through 180°, and then stripped off at 1 cm/min. The peel force Ρ per unit width of the detaching layer was calculated from the timeaverage force observed (10). Results and Discussion Solubility and Composition. The copolymers are quite soluble and continue to be soluble if antioxidant is added; otherwise some gelatin occurs. The usual solvents for neoprene and P T H F ( C H C 1 , C H C 1 , C C 1 , toluene, T H F , etc. ) are also solvents for the graft copolymers. But there are some unexpected features about the solubility. Benzene dis solves both the backbone and the branch quite readily. Yet at least one graft copolymer, which dissolved readily and completely i n C H C 1 and ethyl acetate, swelled but did not dissolve i n benzene. Also some of the copolymers were not completely soluble in toluene and T H F . Still all the neoprene-g-PTHF copolymers were 100% soluble in ethyl acetate, a nonsolvent for neoprene. As shown i n Table III, the ethyl-acetate extracts had in each case the same composition by Ή N M R as the crude products. Thus it appears that the backbone was pulled into its nonsolvent by the P T H F branches. This means that no unreacted backbone remained. 2

2

3

4

2

2

Molecular-Weight Distribution. The solid line i n Figure 1 shows a typical gel-permeation-chromatogram trace for an unfractionated neo prene-g-PTHF (Polymer 1, Table I) that was soluble i n T H F . The


592

MULTIPHASE

Table III.

POLYMERS

Comparison of Composition of Different Samples of Neoprene-g-PTHF Composition Determined by

Sample No.

m NMR on Crude Product

*H NMR on Ethyl Acetate Extract

Weight

2 neoprene PTHF

(%) (%)

68 32

67 33

68 32

3 neoprene PTHF

{%) (%)

38 62

38 62

40 60

6 neoprene PTHF

{%) (%)

10 90

10 90

10 90

7 neoprene PTHF

(%) (%)

87 13

87 13

86 14

polymer is characterized by a unimolecular weight distribution and has a high-molecular-weight t a i l There is no evidence of homopolytetrahydrofuran, which should appear at high count, 50-60, at this low molecular weight. Compared with the original untreated backbone ( dashed curve in Figure 1), the maximum is shifted to higher count (lower molecular weight); but compared with backbone exposed to typical reaction conditions using nonpolymerizable-heterocycle 2-methyltetrahydrofuran i n -

NEOPRENE

G P C

W

COUNTS

Figure 1. Gel-permeation chromatograms of unfractionated Neoprene W-gPTHF. Polymer J , Table I ( ); Neoprene W after reaction with AgPF in 2-methyltetrahydrofuran ( · — · —); Neoprene W before exposure to grafting reaction conditions ( ). 6


30.

L E H M A N N

A N D DREYFUSS


593

Figure 2. DTA thermograms of Neoprene W-gPTHFs. Scan rate = 20°C/min. The numbers on the curves indicate the percent PTHF in the poly mer. stead of T H F , the maximum is shifted to lower count (higher molecular weight). The data indicate that neoprene is significantly degraded under reaction conditions and that, as would be expected, the molecular size of the graft copolymer is increased compared with the treated backbone. Differential Thermal Analysis. The unfractionated graft copolymers showed transitions between those of the homopolymers when examined by D T A . T w o transitions were found for each copolymer examined. As shown in Figures 2 and 3, the TgS decreased and the T s increased as the percent by weight of P T H F increased. Morphology. The D T A results suggest that only one phase might be present in the graft copolymers. However, optical micrographs of a thin film of graft copolymer taken between crossed polaroids show two phases, one spherulitic and the other nonspherulitic. The size of the spherulitic regions varied with the P T H F content. Typical photomicrographs are shown i n Figure 4. Stress-Strain Properties. Figure 5 shows the engineering stress δ i n M P a plotted vs. elongation Ε for solution-cast uncured neoprene-g-PTHF copolymers of different compositions. Curves for Neoprene W and P T H F are also plotted for comparison. The curves show that the elongation at break decreases with increasing P T H F content i n the graft copolymer, the ultimate tensile strength increases with increasing P T H F i n the graft m


594

MULTIPHASE

POLYMERS

copolymer, and elongation and ultimate tensile strength of the graft copolymers lie between those of the homopolymers. Permanent set was calculated after 100% elongation and is given i n Table I V . Permanent set increased with increasing P T H F content i n the graft copolymer. However, if the samples were heated gently, the samples returned to their original shape and the permanent set was lost presumably because of melting of the crystallites formed on stretching.

Figure 4. Optical micrographs of neoprene-g-PTHF with 90, 60, and 14% PTHF, samples 6, 3, and 7 in Table I, respectively. Crossed polaroids at 250 X. The numbers below the micrographs give the % PTHF in the graft copolymers.


L E H M A N N

Table IV.

A N D DREYFUSS


Permanent Set vs. Graft Copolymer Composition PTHF

Permanent Set after 16 hr (%)

(%)

15 50 20 9.5 5

100 90 60 14 0

ι oo

500

I

ο oo

Figure 5. Stress-strain measurements of Neoprene W (A), PTHF (O), and neoprene-g-PTHF with varying PTHF content: 90% (·), 60% (X), 14% (+), samples 6, 3, and 7 in Table I, respectively.


596

MULTIPHASE

POLYMERS

Table V . Adhesion of Neoprene A C S o f t - g - P T H F Copolymers, Neoprene A C Soft, and p X f f F to Various Substrates

Peeling Force on Substrate Listed PTHF (%)

Sample Neoprene go PTHF

b

0 14 69 100

peel tests at 1 AC Soft.

° 180° 6

PPO m

370 310 630

d

(Ή/πι)

a

Mylar

Alumi num

Chrome

Brass

560 760 780 400

1220 840 850 530

170 220 300 120*

480 290 280 520

cm/min crosshead

d

d

d

speed.

See Tables I and II. * A l l samples except chrome failed in a mixed cohesive-adhesive mode. Tests were run 1-2 days after preparation of the test specimens. Since P T H F may crystal lize slowly even in the presence of the compounding diluents, there could be an im portant effect of aging. c

Adhesion Properties. The adhesion properties of two graft copoly mers (numbers 7 and 8 i n Table I ) were studied using several rigid substrates and were compared with the adhesive properties of the homopolymers from which they were derived. The data are given in Table V . Although the peeling force measured at 180°C and 1 cm/min varied considerably with the substrate, it was not very sensitive to the composi tion of the adhesive. As the P T H F content increased, a small decrease in peeling force was observed with P P O , aluminum, and brass substrates. A small increase was observed with Mylar and chrome substrates. Summary. Solubility, Ή N M R , D T A , and G P C all support the con clusion that pure graft copolymer is obtained by the present method of synthesis. The only purification necessary is removal of the silver salts. Properties of the neoprene-g-PTHF copolymers depend on composition and branch length and generally lie between those of the backbone and branch. Acknowledgment This work was supported by the U.S. Army Research Office. W e thank the Ε . I. duPont de Nemours and C o . for supplying the samples of Neoprene W , Neoprene A C Soft, and some of the reagents used i n the adhesion testing. Literature Cited 1. 2. 3. 4.

Dreyfuss, P., Kennedy, J. P., J. Polym. Sci., Polym. Symp. (1976) 56, 129. Dreyfuss, P., Dreyfuss, M. P., Adv. Polym. Sci. (1967) 4, 528. Lee, K., Dreyfuss, P., ACS Symp. Ser. (1977) 59, 24. Quirk, R., Lee, D. P., Dreyfuss, P., unpublished data.


RECEIVED

30. LEHMANN AND DREYFUSS Effect of Branch Length

597

5. Maynard, J. T., Mochel, W. E., J. Polym. Sci. (1954) 13, 251. 6. Cagle, C. V., "Adhesive Bonding: Techniques and Applications," Chap. 5, McGraw Hill, New York, 1968. 7. Fetters, L. J., Morton, M., Macromolecules (1974) 7, 552. 8. Kennedy, J. P., Smith, R. R., "Recent Advances in Polymer Blends, Grafts, and Blocks," L. H. Sperling, Ed., p. 303, Plenum, New York, 1974. 9. Elastomers Chemicals Dept., Elastomers Laboratory, "Neoprene Solvent Adhesives," Ε. I. duPont de Nemours and Co., Inc. 10. Ahagon, Α., Gent, A. N., J. Polym. Sci., Polym. Phys. Ed. (1975) 13, 1285. April 14, 1978.


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