Monomer Interaction - American Chemical Society

Chapter 30

Monomer Interaction Downloaded by UNIV OF SYDNEY on May 19, 2018 | https://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch030

Effects on Polymer Cure 1

2

Jose' A. Ors , Ivan M . Nunez , and L. A. 1

Engineering 2

2

Falanga

Research Center, AT&T, Box 900, Princeton, NJ 08540 AT&T Bell Laboratories, Whippany, NJ 07981

The interactions between the components that make up a photopolymer are extremely important in arriving at a working formulation. Here we show that inclusion of pyrrolidone derivatives like NVP or NMP in acrylate systems enhances the ambient cure of a film. From the reactivity parameters of some simple systems we have derived an empirical scheme for the formulation of fully and/or partially reactive systems based on the molar equivalent ratios of the acrylate to pyrrolidone components. The data support the presence of a synergistic effect between NVP and the acrylate components.

Two of the main considerations in the development of totally reactive liquid photopolymer systems are the resin(s) and the reactive diluents (monomers). The resins play a major role in determining the end properties and therefore the applications of the cured polymer. The reactive diluents are used to provide a fully reactive system with the appropriate reactivity, viscosity, coatability before cure and the desired crosslink density, chemical resistance and dielectric character once it is cured. The photoreactive monomers most commonly used are acrylate based derivatives because of the properties they impart, and their high reactivity and wide solubility range. We focus here on a different type of monomer, N-vinyl pyrrolidone (NVP). This monomer is extensively used in the coating industry to add strength, dye receptivity, hardness, hydrophylicity and improved adhesion to copolymers of acrylate systems. Further note has been made of N V P use because of its low viscosity and its ability to enhance curing. (1-2)

0097-6156/88/0367-0439$06.00/0 © 1988 American Chemical Society Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

CROSS-LINKED POLYMERS

Downloaded by UNIV OF SYDNEY on May 19, 2018 | https://pubs.acs.org Publication Date: April 18, 1988 | doi: 10.1021/bk-1988-0367.ch030

440

This ambient cure enhancement effect agrees with an earlier report (3) where addition of either N V P or N M P (N-methylpyrrolidone), the nonreactive N-methyl analog, showed an increase in the photopolymerization rate of acrylates used in laser disc fabrication. The non-reactivity of the N M P solely refers to its lack of olefinic group hence incorporation into the polymer network via a vinyl moiety. The reduced oxygen effect and high cure rate suggest the possible involvement of charge-transfer assisted polymerization in these neat aery late monomers. (4) Further observations showed that when the cure was effected under nitrogen only a small rate enhancement remained. This suggests that enhancement under ambient conditions is a result of enhanced oxygen consumption by the N V P . (5) The marked contrast of these properties prompted us to investigate the behavior of these pyrrolidone derivatives and to find the correct blend of monomers that will yield the optimal reactivity, cured properties and morphology under the cure process conditions of a liquid negative acting resist. The contrast between N V P and N M P allows us to determine the type of monomer contribution to the cure of the mixtures and differentiate between synergistic reactivity and other contributions such as reduced oxy gen inhibition. The cure process entails the coating of a film ranging from 25 to 100 μπι (wet thickness) followed by a brief off-contact exposure to collimated uv light and an image development step and a subsequent final cure with a high dose of uv light. EXPERIMENTAL A l l the materials in this study are commercially available and were used as received. The compositions of the mixtures are given in Table I. The components can be described as follows: Resin is a proprietary blend of acrylated epoxy resins with an number average molecular weight (Mn) of ~4800, based on acrylated diglycidyl bisphenol A , DGEBAcr is the diacrylate derivative of diglycidyl bisphenol A , IBO A is isobornyl acrylate, TMPTA is trimethyloltriacrylate and DMPA is 2,2-dimethoxy-2-phenyl acetophenone. The infrared data were obtained using a Nicolet FT-IR Spectrometer Model 7199 in a single beam mode. Each data point was an average of two sets of 32 scans. Film samples were irradiated at room temperature through NaCl plates using a filtered Xe/Hg arc lamp source giving an effective wavelength range of 334 ± 20 nm and an intensity of O.SS mW/cm . A l l values for the reaction efficiencies are reported relative to the rate constant for neat IBOA, which serves as a standard. The rate comparison is done by monitoring the following wavelengths: a) an over lap band at 1643-1566 cm ~ corresponding to the C = C stretch band of the olefinic moiety α to the carbonyl with contributions from the vinyl group of the N V P , b) an NVP-band at 840 cm~ , and c) an acrylate-band at 810 cm~ . 2

1

l

l

Dickie et al.; Cross-Linked Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

30.

Monomer Interaction:

O R S ET AL.

Effects on Polymer

Cure

441

Table I. Mixture Compositions


ITUAtUl ^

Resin

Components ( W e i g h t Percent) DGEBAcr IBOA TMPTA NVP NMP*

M 1-2 1-3 1-4 1-5 1-6 1-7 1-8

78.0 68.0 59.0 49.0 39.0 30.0 20.0 10.0

T-l T-2 T-3 T-4

88.0 49.0 29.0 20.0

D-l D-2 D-3 D-4

88.0 69.0 49.0 30.0

NV-1 NV-2 NV-3 NV-4 NV-5 NV-6i

57.3 57.0 57.1 57.0 57.0 48.0

38.0 33.1 23.1 18.1 28.2 32.0

NM-1 NM-2 NM-3 NM-4 NM-5

59.4 59.1 59.4 59.1 59.0

34.1 29.4 19.4 9.5

DMPA

St

20.0 30.0 39.0 49.0 59.0 68.0 78.0 88.0

2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0

0.32 0.45 0.55 0.66 0.74 0.81 0.88 0.94

10.0 49.0 69.0 78.0

2.0 2.0 2.0 2.0

0.91 0.53 0.33 0.22

10.0 29.0 49.0 68.0

2.0 2.0 2.0 2.0

0.79 0.50 0.30 0.15

3.1 8.3 18.2 23.3 13.2 16.0

1.6 1.6 1.6 1.6 1.6 1.6

0.93 0.76 0.54 0.44 0.64 0.48

1.6 1.6 1.6 1.6 1.6

0.82 0.64 0.40 0.22 0.09

4.9 9.9 19.6 29.8 39.4

* A functionality of one is assumed for the non-reactive NMP. The molar equivalent (X) is calculated based on the degree offunctionality divided by Σ (MW) t The parameter S = — — - γ — is used to simplify the plotting of the data. of the acrylate components. Hence, Ε - 0.5 indicates an equimolar ratio of components. $ NV-6: the remaining 2.4% consists of additives like pigment,flowmodifiers, etc.



442

The sol fraction data were obtained from the methylene chloride extraction of films immediately after exposure at ambient conditions with an intensity of 1 mW/cm . (6) The degree of oxygen inhibition was obtained by measuring the thickness of uncured material on the surface ( δ ) of a film exposed in the presence of air with a Hg arc lamp with an intensity output (300-400 nm range) of =*9 mW/cm . Light intensity vari ations were done using optical density filters. Following the initial expo sure the uncured layer at the film surface was wiped with a cloth dam pened with 1,1,1-trichloroethane (TCE). The still soft film was then fully cured with a dose of =*2.8 Joules/cm . The thickness difference between the wiped and unwiped portions of the film was then measured with a Dektak surface profilometer. 2

θ 2

2


2

R E S U L T S and D I S C U S S I O N Our preliminary formulation work using N V P agreed with reported obser vations of cure enhancement using this monomer under ambient condi tions. To investigate the component interactions we decided to examine the behavior of the neat monomers followed by the interactions of monomer-NVP, resin-NVP, and finally more complex mixtures containing either N V P or N M P . Monomers. Figure 1 shows the contrast in FTIR curing profile for the neat materials. The component reactivity were based on the first order rate constants of the initial slopes of the disappearance of the respective olefinic moieties. The results T M P T A (3.4) > IBOA (1.0) > D G E B A c r (0.7) > N V P (0.14) show that the neat monomers follow the expected cure profile vinyl < acrylate under similar conditions (e.g. photoinitiator concentration, light intensity, etc.). Even though a decrease in the extent of conversion with longer irradiation times is observed in the higher viscosity D G E B A c r resin, its initial rate is faster than the N V P . (7) The change in slope follows the two-regime cure profile observed in more complex systems. (8) The slow cure is in sharp contrast with the observed rate enhancement when this pyrrolidone derivative is mixed with acrylates. Monitoring the individual reactivity by using the distinct ir frequencies for the various components was used to quantify the controlling parameters and lead to improved formulation schemes. Acrylate Monomer/NVP. To observe the interaction between monomers, we compared the behavior of two monomer mixtures, I and T. These mix tures contrast the interaction of N V P and a monofunctional acrylate (IBOA), which should yield to linear polymerization, with a afunctional monomer (TMPTA) which results in a crosslinked network. The FTIR cure profiles of some I mixtures using the overlap and the acrylate bands (Figure 2) show a fast initial rate followed by a slow down with extended irradiation times. The change in cure regime takes place at


30.

ORS ET AL.

Monomer

Interaction:

Effects

on Polymer

Cure 4 4 3

S 7 / J S 7 A

Ο

a k a

NVP DGEBAcr TMPTA IBOA

7

% Conversion 6θ}~!


[M)/[Ref]

100

200

300 400

500

600

Time (sec)

Figure 1 Comparison of Monomer Reactivity. Cure Study - FTIR Data

Figure 2 C u r e S t u d y I M i x t u r e s - F T I R P r o f i l e . O v e r l a p B a n d (left), A c r y l a t e B a n d ( r i g h t )



444

different conversion levels (1-4 > 1-7 > 1-8) which appear to vary inversely with the N V P concentration (1-4 < 1-7 < 1-8). The subsequent slope approximates the rate of the neat N V P . Monitoring the acrylateband shows an enhancement in the initial IBOA rate that appears to be proportional to the N V P concentration. The slope change is also observed with extended irradiation times, but in the N V P rich mixtures a high acrylate conversion level (^ 80%) before any slow-down is noticeable. To interpret these results the initial rate constants (relative to IBOA), for both the overlap band and the acrylate moiety were plotted against a molar equivalent ratio parameter, Ξ , in the form of S = XAcr/( Acr + * # ) · Figures 3 and 4 compare the changes in reactivity with compositional variation of the overlap rate with the acrylate and N V P components respectively. The profiles show that the reactivity of each functional group increases as Ξ - 0 . 5 (X /X -1), followed by a decrease in the composite rate down to the value of the neat N V P as the ratio decreases ( Ξ - 0 ) . In contrast, the IBOA reactivity appears to reach a pla teau at the higher N V P concentrations indicating a selective cure pattern arising from the reactivity difference of each component, leading to possi ble heterogeneity in the final film. (9) The heterogeneous cure can result in a mostly poly-NVP film that can either contain grafted segments of IBOA-NVP copolymer and/or poly-IBOA in its matrix. A similar dual rate behavior is found for the trifunctional T M P T A in the Τ mixtures, again monitoring the overlap band shows an optimum rate as Ξ - 0 . 5 , Figure 5. The T M P T A emphasizes the equivalency dependence on the acrylate moiety and supports an earlier report with this monomer combination. (10)


x

Acr

N

Resin/NVP Mixtures. Since acrylates show similar cure profiles regardless of functionality, the next step was to investigate the curing behavior of the D G E B A c r resin with N V P . Here, series D , the informa tion regarding the N V P component is difficult to obtain since the resin exhibits an absorption band at 834 c m " which interferes with the 840 c m " N V P band. Comparison of the curing profiles of these mixtures with the neat D G E B A c r show the expected enhancement in the reaction rate along with an increase in the extent of conversion at the higher N V P concentrations (Figure 6). The combination of lower viscosity and enhanced reactivity significantly improves the through cure of the film and supports earlier reports where the weight percent N V P ranged from 15 to 25% of the resin system still below the molar equivalency. The sol fraction data in Figure 7 show a decrease in extractables as Ξ-0.5 followed by an increase in residual unreacted material at higher N V P concentration under the same exposure dose. Gas chromatographic analysis showed that the N V P becomes the main component of the soluble fraction, in the solvent extraction, particularly at the higher initial concen trations. This supports the earlier observation on heterogeneity of cure, 1

1


30.

Monomer

ORS ET AL.

Interaction:

Effects

on Polymer

Relative Reactivity


0

0.25

0.5

0.75

1

Figure 3 I M i x t u r e s - Relative Reactivity versus M o l a r E q u i v a l e n t R a t i o .


Cure

445


446


where the acrylate portion cures preferentially leaving unreacted N V P in the film. Complex Mixtures. In the ternary mixtures N M and N V (Table I) the contrast between the reactive versus non-reactive pyrrolidones should help clarify some of the specific interactions of these derivatives with the acrylates. Since the oxygen concentration is limited to the dissolved oxy gen (11-12) , because of FTIR sample configuration, we presume that comparison of the rates yields the enhancement difference between N V P and N M P . Figures 8 and 9 show the reactivity contrast between the two sets of mixtures. While no significant change in relative reactivity is observed in the N M series, the N V mixtures show the previously noted enhancement trend. The slight increase in the acrylate reactivity of NM-S could be attributed in part to changes in the polarity of the medium, caus ing a possible selectivity in the affinity of the acrylate groups and even an agglomeration of acrylate groups. Spectroscopic data (uv-vis) gave no evi dence for ground state complex formation between the N V P and IBOA. Solvent extraction data (Figure 10) from a series of N V films, irradi ated under ambient conditions, show the expected reduction in the sol fraction with increasing N V P concentration even at values below molar equivalency (Ξ = 0.44). The effect of oxygen concentration is apparent when contrasting these results with the optimum reaction ratio of 0.5 obtained from the FTIR data of the overlap band, under limited oxygen concentration conditions. In contrast, the extraction data for N M films show a steady increase in the sol fraction. However, the values do not correlate directly with the amount of N M P in the initial formulations. This observation along with the lack of reactivity enhancement, under con trolled environment, imply involvement in the reduction of the oxygen effect that lead to a higher extent of acrylate reaction. This effect should be more evident at the surface of the film, where the oxygen concentration is higher, and therefore lead to a reduction in the thickness of the inhibi tion layer ( δ ) . The inhibited surface appears as an uncured (wet) layer on the surface of the partially cured film whose thickness is dependent on the incident light intensity (/ ), the exposure time (f ), and the photoinitiator concentration ([PI]) according to equation (1) θ 2

0

MV

A + Blt

uv

δ

ο

' -

h [PI]

( 1 )

where A and Β are empirically obtained constants, specific for the light source, photoinitiator and monomer/resin systems used. Figures 11 and 12 exemplify the δ # dependence on the light intensity and the exposure time parameters for an N V P containing mixture. Figure 13 shows the reduction in the thickness of the oxygen inhibi tion layer at two different irradiation doses (45 and 540 mJ/cm ) for both series. As expected the effect is more pronounced in the N V series and 2

2



ORS ET AL.

0J


0

Effects on Polymer

1

I

I

0.25

0.5

0.75

Cure 4 4 7

L_

1

Figure 6 D M i x t u r e s - R e l a t i v e R e a c t i v i t y versus M o l a r E q u i v a l e n t R a t i o . (•) Exposwedose 15 mJ/cm 402

30%SOL

20100

0.25

0.5

0.75 5D

Figure 7 D M i x t u r e s - S o l F r a c t i o n versus M o l a r E q u i v a l e n t R a t i o .

(a) Overlap Bud (O) Acrylate Bud

4 Relative Reactivity

3— 2 10

0.25

0.5

0.75

Βλ*

Figure 8 N V M i x t u r e s - R e l a t i v e R e a c t i v i t y versus M o l a r E q u i v a l e n t R a t i o .

American Chemical Society Library 1155 16th St, N,W, Dickie et al.; Cross-Linked WisMutfon, D £ Polymers 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

448


6

(•) Overlap Band (O) Acrylate Band

5-1 4 Relative Reactivity

3— 2 1-


0

0.25

0.5

0.75

Figure 9 NM Mixtures - Relative Reactivity versus Molar Equivalent Ratio. (•) NMMixtura (•) NV Mixture

605550%SOL

4540353025-

0.25

0.5

0.75

Figure 10 Sol Fraction versus Molar Equivalent Ratio. Exposure Dose: 60

2

mJ/cm .

0.7 - , 0.6 0.50.4-

δθ2

(mils)

0.30.2-

• • 0.45

0.10

0.05

4-

-L

0.1

0.15

0.2

l/ί,,ν (sec' ) 1

Figure 11 Dependence of Oxygen Inhibited Layer ( b ) on Exposure Time (r*v). Q2




ORS ET AL.

Effects on Polymer

Cure 4 4 9

Figure 12 Dependence of Oxygen Inhibited Layer ( h ) on Light Intensity ( / ). Q2

0.9-

0.7-4 Exposure dose:

45 mJ/cm

2

0.5 H 0

0.3-4

a

„- ~

(•) NM Mixtures (·) NV Mixture (•) NM Mixture (O) NV Mixtura

Exposure dose:

540 mJ/cm

1

0.1-

0.25

0.5

0.75

Figure 13 Variations in h with Component Equivalent Ratio. Q2


1

450

CROSS-LINKED

POLYMERS


can to be attributed to a combination of oxygen scavenging with increased component reactivity. The N M mixtures exhibit a similar trend except for a marked increase in the thickness of the inhibited layer at Ξ = 0 . 2 2 with the low exposure dose. The latter may be attributed to a dilution effect that when coupled with the selective increase in acrylate reactivity indicates heterogeneous cure. Reactivity Enhancement. The preceding data suggest that the cure of these acrylates is enhanced by the presence of the pyrrolidone derivatives. Two major pathways appear to be operative, which are dependent on the concentration of oxygen and the nature of the pyrrolidone derivative. First, in the absence or at low concentrations of oxygen, the contrast in initial rate enhancement of the N V versus the N M mixtures could be mostly attributed to the interaction of the acrylate and N V P , since no sig nificant variation was noted with N M P (Figure 8). The rate increase with added N V P can, in turn, be related to the reactivity of the monomer and general solvent effects such as the polarity of the medium, the reduction in viscosity, improved miscibility, etc. Secondly, in the presence of higher concentrations of oxygen (eg. Ambient conditions) the rate enhancement can be a result of the reduction in the oxygen inhibition observed with increasing N V P or N M P concentration. The effect could involve a combi nation of solvent effects and/or a tertiary amine-type behavior by the lac tam in scavenging the oxygen in the film as described by Roffey. (13) It appears that in the N V mixtures both of these pathways are operative. Furthermore, hydrogen abstraction needed to generate the relatively unreactive peroxide can lead to formation of active radicals that could be involved in the polymerization sequence. Lactam-Benzoin Initiator Interaction. We have shown that whenever one of these pyrrolidone derivatives is present, the ambient cure is enhanced. A variety of reasons can be responsible, including the ones proposed above, however a possibility not discussed is the interaction between an excited state of the photoinitiator and the lactam. Spectroscopic data showed no changes in the D M P A absorption spectrum with addition of NVP. This initiator (DMPA) is particularly effective because of its dual fragmentation nature, after the initial Norrish-I type photofragmentation (14) the generated dimethoxybenzyl radicals can further fragment to yield a methyl radical and methylbenzoate. However, the initial radicals can be assume come from the triplet (T\) state of the excited initiator. (15) This triplet could form an exciplex or ion pair with the pyrrolidone leading to radical formation through a hydrogen abstraction of the latter. (16) This pathway can compete with oxygen inhibition thus showing an increase in acrylate reactivity when these lactams are added. However, since we have shown that the D M P A initiated polymerization of N V P is slower than of


30.

ORS ET AL.


Effects on Polymer

Cure

451

the acrylate components used in this study any complex formation between the D M P A and the N V P can not solely account for the enhanced rate. SUMMARY


As discussed at the onset, this study was focused on determining the reactivity interaction of two lactam derivatives with acrylates in a variety of systems ranging from simple monomer to more complex mixtures. The definition of such interplay allows us to derive a simplistic scheme for the formulation of fully reactive blends containing N V P as a reactive monomer. The data show that: 1. A n enhancement effect exists between N V P and the acrylate components in the cure of these systems. This behavior is interpreted as synergistic effect resulting from a variety of complex interactions between the components and not simply as a solvent effect. 2.

Homogeneity of the cured films can be maintained in these systems as long as 0.5 (X /X ^l). Of course, the optimum ratio will depend on specific components and their interactions. Acr

N

3.

A t high concentrations of N V P , B

Monomer Interaction - American Chemical Society

Recommend Documents