Inverse Gas Chromatography of Polymer Blends - ACS Symposium

Chapter

10

Inverse Gas Chromatography of Polymer Blends

Downloaded by GEORGE MASON UNIV on December 23, 2014 | http://pubs.acs.org Publication Date: April 24, 1989 | doi: 10.1021/bk-1989-0391.ch010

Theory and Practice 1

2

Mohammad J . El-Hibri , Weizhuang Cheng , Paul Hattam, and Petr Munk Department of Chemistry and Center for Polymer Research, University of Texas, Austin, TX 78712 With careful experimental design, inverse gas chromatography can be a viable method for the determination of the polymer-polymer interaction coefficient B . The variation of apparent B values with the probe is shown to be related to the chemical nature of the probe and not due solely to experimental error. A method is presented to allow the estimation of the 'true' B value. Experiments were performed on a 50/50 blend of poly(epichloro-hydrin)/poly(Єcaprolactone) at several temperatures. Polymer and blend solubility parameters were determined. 23

23

23

Probing polymer-polymer interactions i n miscible blends i s an experimentally d i f f i c u l t task. Most methods available for this purpose are elaborate and limited i n their applicability. In recent years, research has shown that inverse gas chromatography (ICC) offers great promise for the study of polymer-polymer interactions. Conceptually, the technique involves the following: the elution behavior of volatile organic compounds (probes) i s measured for one or more blend columns and compared with the retention behavior of two homopolymers studied under identical conditions. An excess retention can then be characterized and treated as a measure of polymer-polymer interaction strength. This polymer-polymer interaction i s the cause of the irascibility phenomenon and i s of practical interest. Earlier attempts at using IGC to characterize polymer blends were unsuccessful. The polymer-polymer interaction parameters evaluated were found to vary with the probe used (1-5). For this reason, the use of IOC for the study of blends has been severely 1Current address: Amoco Performance Products, P.O. Box 409, Bound Brook, NJ 08805 Current address: Department of Materials, Building Materials College, Shanghai, People's Republic of China 0097-6156/89/0391-0121$06.00/0 1989 American Chemical Society

2

β

In Inverse Gas Chromatography; Lloyd, D., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

122

INVERSE GAS CHROMATOGRAPHY

neglected. Given the importance of polymer i n t e r a c t i o n data and i t s u n a v a i l a b i l i t y through other methods, a thorough investigation of the technique was undertaken. A refined methodology f o r obtaining the experimental data of i n t e r e s t was implemented. This new methodology, i s i n part, the subject of t h i s paper. The system poly(€-caprolactone) /poly(epichlorohydrin) (PCL/PBCH), a known compatible blend, was studied over the temperature range of 80 t o 120 C. (For the comparison of data, r e s u l t s from an e a r l i e r work (6) on blends of composition PCL/PBCH 25/75 and PCL/PBCH 75/25 measured a t 80°C have been included). Twenty-five probes, representing a number of chemical f a m i l i e s , were used to examine the chemical contribution of the probe to the apparent value of the polymer-polymer i n t e r a c t i o n parameter derived from the IGC data.


#

Theory The e l u t i o n behavior of a probe on an IGC column i s routinely described by the s p e c i f i c retention volume Vg , defined as (V

V = g

r

- V )/w

(1)

= Vj/W

Q

where V i s the probe e l u t i o n volume, V i s the void volume of the column, V i s the net retention of the column, and w i s the mass of the polymer. Combining the Flory-Huggins theory with standard chromatographic c a l c u l a t i o n s , the probe-polymer i n t e r a c t i o n parameter X i 2 can be written as (2) r

Q

n

X

= In (RTv /V ViPi) - 1 + ViA*zV2 '

1 2

2

(%1

g

v

" l

(2)

JPL/KT

In Equation 2, V i and v are the probe molar volume and polymer s p e c i f i c volume, respectively; M i s the polymer molecular weight and R i s the gas constant. P]* i s the probe vapor pressure and B n i s i t s second v i r i a l c o e f f i c i e n t i n the gas phase. For work with high polymers, the t h i r d term of Equation 2 becomes n e g l i g i b l e and may be omitted. G u i l l e t and coworkers (8-10) have determined the s o l u b i l i t y parameter of polymers from the probe-polymer i n t e r a c t i o n c o e f f i c i e n t s . They separated the i n t e r a c t i o n parameter i n t o entropic and enthalpic contributions, such that X I 2 X H + X S t o y i e l d , i n combination with Hildebrand's solution theory, the following expression; 2

2

=

X

= v i ( 6 i - 8 ) /RT +X 2

1 2

2

(3)

S

where hi and & are the solvent and polymer s o l u b i l i t y parameters, respectively. By expanding the expression i n parentheses, they obtained the l i n e a r expression 2

c^RT

-X12

Ai

=tth2/mZ>i- (C2 /RT -Xs/Vl) 2

4

The experimental value of the l e f t of Equation 4 was plotted against &[ . A value of 6 and an average value of %s^l ^ obtained by regression a n a l y s i s . This method can be applied to blends, considering the blend as r

2


e

10.

E L - H I B R I ET AL.

123


a single component and having a s o l u b i l i t y parameter 823; the i n t e r a c t i o n c o e f f i c i e n t being written asX],(23)following the G u i l l e t approach we d i d not separate X12 i n t o entropic and enthalpic components. Instead, a more general term, C I ( 2 3 ) J was introduced to represent a combination of i n t e r a c t i o n terms. By introducing the contact energy per u n i t volume, B^(23), a simpler form of the expression i s obtained; 1 x 1


B

l(23) =

K r

Xl(23)

= (5i - 0 2 3 )

2

+ Ci(23)

1

= 2S2301 - hl

- Ci(23)

3

(6)

Thus, a p l o t of - 3^23) versus 81 y i e l d s 2823 from the slope and an average value of C^(23) the intercept. When using IGC f o r the evaluation of the polymer-polymer i n t e r a c t i o n c o e f f i c i e n t X23 / the free energy of mixing i s routinely expressed by an extension of the Flory-Huggins expression (11) t o a three component system (12); f

G

^ mix

=

r

o

m

KTJnxlitfx + n ln#2 + r ^ l n ^ + 1 ^ 2 * 1 2 2

+ n ^ 3 K i 3 + 1^/^23]

(7

>

where , a n d X i j are the number of moles, volume f r a c t i o n , and binary i n t e r a c t i o n parameter, respectively. An alternate parameter X 2 3 \ related t o X23 as X 3'

= (Vi/V )X 3

2

2

2

i s conventionally used to describe the polymer-polymer i n t e r a c t i o n term as i t removes the rather large value of the molar volume of the polymer, V . Routine thermodynamic calculations y i e l d the expression f o r X 3 » 2

2

A

S

= (V^3){ln[Vg /(W2V2+ /b

V%V3)] -

&ln(V , /V2) g

2

(9)

-/rf ln(%3/Y3>} 3

where the subscripts of Vg r e f e r to the blend and t o the homopolymers. W2 and W3 r e f e r t o the weight f r a c t i o n s of the two polymers i n the blend. The i n t e r a c t i o n parameter may be given i n terms of the contact energy per unit volume of the blend using the quantity B 3 , i n which X23 i s normalised with respect t o the probe molar volume V : 1

2

x

B

2 3

= RrX 3'/vi 2

B23 i s expected to be independent of the nature of the probe.


do)

124

INVERSE GAS CHROMATOGRAPHY


Improvements i n the IGC Method I t was recognized that the l e v e l s of p r e c i s i o n and r e p r o d u c i b i l i t y adequate i n IGC studies of homopolymers were inadequate f o r a successful study of blend systems. A column-to-column r e p r o d u c i b i l i t y of 1% was deemed necessary f o r t h i s purpose. This i s because the quantity of i n t e r e s t i n the case of blends i s the difference between the retentions of the blend column and the homopolymer columns, which i s usually l e s s than 10% of the observed retention f o r any of the i n d i v i d u a l columns. Thus, a number of experimental and data analysis improvements has been introduced to the technique, which have boosted the r e p r o d u c i b i l i t y of the data considerably. Experimental Modifications. Perhaps the most s i g n i f i c a n t change introduced i s the mode of coating the polymer onto the i n e r t packing. T r a d i t i o n a l l y , the polymer sample i s deposited onto the support i n solution, using slow solvent evaporation. This method has the disadvantage of preventing precise determination of the polymer mass due t o losses of polymer on the walls of the preparation vessel. Calcination and Soxhlet extraction, performed f o r subsequent mass determination, have been shown t o be major causes of error (13,14). We used a new coating technique, ( p a r t i a l soaking method), which consists of the following steps. The polymer i s f i r s t dissolved i n a suitable solvent. The support i s then p i l e d on a watch glass and a portion of the solution added to the top of the support p i l e . Care i s exercised so that the s o l u t i o n does not come i n contact with the watch glass. The support i s thoroughly mixed and the process repeated u n t i l a l l the solution has been used (including several rinsings of the solution f l a s k ) . Consequently the exact mass of the polymer coated onto the support i s known. The procedure has been described i n d e t a i l elsewhere (15). Two other experimental aspects were modified. The p r e c i s i o n i n measuring the c a r r i e r gas flow rate was enhanced by a new soap-bubble flow meter design (16). Also, the resolution of the detection of the e l u t i o n data was improved by implementing a custom-configured computer-based data handling system. In t h i s scheme, an HP-3478A d i g i t a l voltmeter was interfaced with a ndcrocomputer using an IEEE-488 interface board (National Instruments) and the detector output monitored. This configuration allows e l u t i o n data to be measured with a signal-to-noise r a t i o of 5 x 1 0 i n the detector output reading. E l u t i o n times are measured with a p r e c i s i o n of ±0.1 s. 4

Data Analysis. A t a c i t assumption that the support material contributes l i t t l e or no retention t o the observed retention by the polymeric coating i s usually made i n the IGC l i t e r a t u r e . In a published work (17), and from a large body of recently gathered data, i t has been confirmed that retention by the so-called i n e r t support may a c t u a l l y account f o r up to 10% of the observed retention of the column. Furthermore, the support retention was found to be a function of the amount of probe injected, e s p e c i a l l y f o r strongly polar probes. I t became c l e a r that t h i s factor alone could undermine the blend analysis i f i t were not handled properly.


10.

EL-HIBRI E T AL.

US


A procedure was developed i n which the retention by the polymer was obtained by subtracting the retention of the support from the observed retention of a given column V^ . According t o t h i s treatment, the s p e c i f i c retention volume i s given by 03

Vg = V g °

b S

U

- ^ P/W

(11)

s u

where V P i s the retention volume of the support, as obtained from an independent experiment on an uncoated support column under i d e n t i c a l conditions. The fundamental assumption made i n Equation 11, the a d d i t i v i t y of the support and polymer retentions, i s strongly supported by our experimental data. The concentration dependence of the support retention f o r the various probes followed the r e l a t i o n


n

InV sup =

Inverse Gas Chromatography of Polymer Blends - ACS Symposium

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