Structure Property Relationship of Polymer Blends: Effect of Grafting on Mechanical Properties

 

B. Sehgal1* and G.B. Kunde2

1Maharaja  Sayajirao University  of Baroda, Department of Applied Chemistry, Vadodara: 390001, Gujrat, India

2Datta Meghe College of Engineering, Dept. of Applied Chemistry, Navi-Mumbai, Maharashtra, India;

*Corresponding Author E-mail: kundebg@yahoo.com

 

ABSTRACT:

The behavior of polymer blend depends on the properties of the individual components in the blend, the proportion in which they are present in blend and the structure and morphology of the blend. The structure of the dissolved molecules of the blend in the polymer-polymer mixture plays a major role in the molecular state of dispersion, the adhesion between the phases and consequently influences most of the properties and applications. In the present study we have made use of grafting of a polymer on to the natural rubber backbone leading to chemical modification of the poly-isoprene allylic double bond system. This type of modification leads to a change, not only in chemical properties but physical properties as well, like increased hardness, strength with decreased tackiness and the blend behaves like a thermoplastic rubber without making use of vulcanization. In order to optimize the processing conditions, the properties like miscibility characteristics, torque and melt viscosity were studied.

 

KEYWORDS: PVC (Poly vinyl chloride), NR (Natural Rubber) ENR (epoxidised natural rubber), PMMA (Poly-methyl methacrylate).

 


 

INTRODUCTION:

Polymer blend is a combination of two or more polymers resulting from a common processing step without the formation of chemical bonds between the components. A complete homogeneous polymer blend averages the properties of two polymers constituting the blend. In case of incompatible blends, each phase tries to maintain the properties of pure homo polymer for example each phase shows its characteristic Tg, but later due to polymer attraction the two Tg’s shift towards each other and broadens. Some amount of compatibility is seen during lowering of the melting point1 when a crystalline polymer is blended with another polymer, it was noted that most polymers that are compatible are polar. Kern2 suggested that even for polar polymers, proper orientation is required to achieve compatibility. Polymer blends that exhibit compatibility at lower temperature may exhibit phase segregation at high temperature. According to Bohn3the polarity of the polymers must be nearly same to yield molecular mixing. Compatibility can be induced in case of incompatible polymers by co polymerization or by addition of a third polymer having intermediate properties of that of a blend.

 

Copolymerization can affect both crystal structure and crystallization. The tensile strength of the polymers fall rapidly as the co monomer concentration increases4,5.

 

The tensile strength can be increased by various modifications6,7 that can be brought in a copolymer. The route of modifying one of the components can either result from bond rearrangements without the inclusion of new atoms like cyclization, cis –trans isomerization or by introduction of new chemicals groups entering from olefinic double bond via addition or substitution reactions. The third modification can be brought about by grafting of a second polymer on to the main chain polymer backbone leading to its chemical modification.

 

In general the miscibility in polymer blends is achieved because of various types of  intermolecular forces eg. Donor-acceptor, dipole–dipole, ion-dipole. 8-10 A donor acceptor type of interaction is seen during the miscibility of polyvinyl chloride with poly (η-caprolactone)11. Studies have shown that PVC has a great affinity towards ENR, especially when the epoxidation level12,13 is about 50 mol%. Blending of PVC with such a component that could increase the tensile strength gave insight into various possibilities like polyesters, poly (η- caprolactone)14,15 etc. Structure property relationship gives an insight into rheological, viscoelastic behavior, degradation characteristic and a route of optimization of processing conditions for their use in industry.

 

MATERIALS AND METHOD:

All the solvents were distilled before use and stored. Malaysian Rubber Association donated NR. Hi Media Lab. supplied MMA. Azoacid chloride was purchased from B.D.H. chemicals. 9 – Fluorenyllithium and 1, 2 dimethoxy ethane were purchased from Aldrich. The present work utilizes azodicarboxylate functionalized PMMA that was grafted on ENR and later melt blended with PVC. To prepare PMMA grafted blend the first step was polymerization of MMA. Anionic polymerization16-20 of methyl methacrylate was carried out in highly solvating media, initiated by 9- fluorenyllithium in 1, 2 dimethoxy ethane at – 20ºC using inert atmosphere. To this functionalized polymer a small amount of azo acid chloride was added leading to azodicarboxylate functional polymer. In order to modify rubber, the rubber particles in latex were epoxidized to 50% following method of D.R. Burfield 21 and the epoxide content was determined by Differential Scanning Calorimeter22 and Infra Red spectroscopy technique23,24.

 

Direct mixing of azodicarboxylate functional Polymethyl methacrylate (5% by wt of PVC) with ENR at 120 ºC maintaining the rotor speed at 60 rpm for 5 minutes was carried out in the Haake Torque Rheocord (90) leading to grafted ENR. Similar blending of PVC with grafted ENR was performed in a Rheocord for different mixes at 100-120oC for six minutes. The molten mass was then removed for casting sheet in a two roll mill. The sheeted out mass was compression molded at 100ºC -140ºC.Various blends that were prepared were ENR, PMMA Grafted ENR,  50% PVC + 50% ENR ,50% PVC + 50% Grafted ENR.

 

For miscibility characteristics DSC measurements were run on DU PONT Differential Scanning Calorimeter (model 9900) in nitrogen atmosphere. Tg’s were taken as the midpoint of the step in the scan while maintaining a heating rate of 20ºC/min. Dynamic mechanical analysis was done using DU Pont DMA (model 983) at a strain amplitude of 0.0025cm and a frequency of 3.5 Hz along with cooling at 100ºC by liquid nitrogen and recording the measurement while the temperature is rising at 2ºC/min.

 

Tensile testing was done at 25±2ºC as per ASTM D 412 –80 test method using a dumbbell shaped sample in Zwick Universal. Tensile Testing Machine (UTM) (M1435) at a cross head speed of 500 mm min-1.

 

RESULTS AND DISCUSSION:

Keeping the parameters like temperature, rotor speed, mixing time and the volume of the mixing components constant, the torque was measured up to 6 minutes mixing. As evident from (Table 1), the grafted blends showed an increased value of torque probably because of increased cross linking leading to reduced segmental mobility. This trend was opposite to non grafted blends where torque got reduced due to chain scission. Stock temperature also showed a jump in final mixing temperature as compared to rigid blends. This further proves that the higher melt viscosity that led to higher stock temperatures must have resulted from cross linking that occurred in grafted blends. A single Tg value in all the samples showed a through mixing in all the samples and thus showed single phase morphology25,26. The grafted blends as seen from (Fig 1) showed a lowering in storage (E′) modulus at the glassy zone. Hence a marginal shift was noted for the damping or tan-δ values. A salient feature as seen from (Fig 2) was the narrowing of the damping peak with grafting. This can be explained by the fact that the structure grafted on ENR further enhances the miscibility in the micro heterogeneous portion if any in the blend.

 

Table I: Effect of Blend Composition on Torque of PVC-Grafted ENR Blends

Time of Mixing in min

Torque in Nm

50% PVC+50% ENR

50% PVC +50% Grafted ENR

(P50)

P50 (G)

Start

20

21

1

18

18.5

2

15.5

17

3

15

16.5

4

14

16.2

5

12.5

15.5

6

12

15.5

 

Fig.1 –Variation of storage Modulus with temperature using various  composition of PVC-ENR (with and without grafting).

 

While the non grafted blends showed rubber like deformation with high elongation at break, the grafted blends showed not only a reasonable increase in tensile strength but also elongation before ultimate failure as shown in (Table 2).The higher tensile strength of the grafted   ENR blends can be reasonably attributed to the occurrence of crystallization of ENR chains under high strain27. As compared to non grafted blends, the proportion of material which could not contribute to an elastic network and which might not experience the full strain required to induce strain crystallization was less in grafted blends.

Table II: Tensile Properties of PVC-Grafted ENR Blends

S. No

Sample Name

Symbol Used

Tensile Strength

% Elongation                                                                                                 at Break

1

ENR

P0

1.5

300

2

Grafted ENR

P 0(G)

4.5

360

3

50% PVC+50% ENR

P50

10

640

4

50% PVC+50% Grafted ENR

P50 (G)

15

710

 
Fig.2 –Variation of damping with temperature using various composition of PVC-ENR (with and without grafting).

 

CONCLUSIONS:

All the blends showed single phase morphology and the structure grafted on ENR further enhances the miscibility in the micro heterogeneous portion present in the blend. While on one side, rigid blends showed more segmental mobility with less torque, the grafted blends showed an increased initial and final torque values along with high tensile strength and elongation, reason being strain crystallization.

 

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Received on 04.02.2010        Modified on 22.02.2010

Accepted on 12.03.2010        © AJRC All right reserved

Asian J. Research Chem. 3(3): July- Sept.  2010; Page 637-639