Y Medvedevskikh - Kinetic model of photoinitiated copolymerization of monofunctional monomers till high conversions - страница 1

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CHEMISTRY & CHEMICAL TECHNOLOGY

Vol. 3, No. 1, 2009 Chemistry Yuriy Medvedevskikh, Galyna Khovanets' and Iryna Yevchuk

KINETIC MODEL OF PHOTOINITIATED COPOLYMERIZATION OF MONOFUNCTIONAL MONOMERS TILL HIGH CONVERSIONS

Department of Physical Chemistry of Combustible Minerals L. M. Lytvynenko Institute of Physico-Organic Chemistry and Coal Chemistry NAS of Ukraine 3a Naukova str., 79053 Lviv, Ukraine vfh@org.lviv.net

Received: July 07, 2008

© Medvedevskikh Yu., Khovanets' G., Yevchuk I. 2009

Abstract. Regularities of kinetics of photoinitiated copolymerization till high conversions in the systems of monofunctional methacrylate comonomers (hydroxyethyl methacrylate (HEMA), glycidyl methacrylate (GMA)) have been investigated by laser interferometry in a wide range of experimental factors (molar ratio of comonomers, photoinitiator concentration, intensity of UV-irradiation). Kinetic model of photoinitiated copolymerization of methacrylates till high conversions has been proposed on the basis of microheterogeneity conception of the polymerization process.

Keywords: kinetic model, copolymerization, photoinitiator

1. Introduction

Copolymers on the basis of various by nature and functionality acrylate and methacrylate monomers are of increasingly wide use in different industries of national economy. It causes a great interest for investigation of copolymerization processes of acrylates and methacrylates. In particular, a lot of researches are devoted to polymerization processes which take place at UV-irradiation since copolymers obtained by photochemical technologies are used in polygraphy for manufacturing of printing boards and plates, integral schemes in radio- and microelectronics, optical discs, holograms, as organic basis of stomatological materials etc. [1, 2]. Most of these works have experimental character and deal with the influence of the acrylate monomers structure and reactive medium nature on kinetics of their polymerization and copolymerization as well as on properties of the obtained products. For example, in [3-4] it was shown that nature of solvent, viscosity and pH of reactive medium, temperature, pressure etc. significantly affect the value of copolymerization rate constants. Some authors [5-8] investigated the influence of additives (diluents, stable radicals, fillers) on kinetic features of photopolymerizing acrylate systems.

However, statistically significant data from kinetics of photoinitiated copolymerization till high conversions, which would serve as an experimental base for kinetic model of the process derivation, are practically absent.

Regularities of kinetics of photoinitiated copolymerization for the initial stage of the process, that is at conversions, when polymerizing system remains homogeneous and is monomer-polymer phase, may be satisfactorily described by the known Mayo-Walling equation. Theoretical description of copolymerization kinetics till high conversions seems to be more complicated, since it is necessary to take into account the division of the system at microphase level.

According to the conception of microheteogeneity of polymerizing system [1, 9] the process is assumed to pass till high conversions in three reaction zones: in monomer-polymer phase, in polymer-monomer one which precipitates from monomer-polymer phase in the form of micrograins, and in the interphase layer. On its basis kinetic models for homopolymerization of mono- and bifunctional monomers have been obtained.

As the development of this direction in the present work kinetics of photoinitiated copolymerization of monofunctional methacrylates till high conversions has been investigated. On the basis of above-mentioned conception the kinetic model for quantitative description of these processes have been suggested.

2. Experimental

2.1. Initial Substances

For experimental researches the following materials were chosen:

2,3-epoxypropyl (methacrylate glycidyl methacrylate) (GMA)

CH2=C-C(O)-O-CH2-CH-CH2

CH3 O

reagent grade (Fluka) with the following characteristics: assay > 97 %, FW 142.16, d20 1.075, n20 1.450, bp 465-470 K;

2-hydroxyethyl methacrylate (HEMA)

CH2=C(CH3)-C(O)-0-CH2-CH2-OH

reagent grade (Aldrich) with the following characteristics: assay > 97 %, FW 130.14, d420 1.073,

nd20 1.453, bp 330-345 K.

Monomers were purified by mixing them with activated Al2O3 powder and subsequent centrifugation [1].

2,2-dimethoxy-1,2-diphenylethane-1-on (IRGACURE 651) reagent grade (Fluka), assay > 98 %, FW 156.30, d420 1.210, mp 330-344 was used as photoinitiator.

2.2. Experimental Technique

Stationary kinetics of photoinitiated copolymerization of methacrylates to high conversions was studied by laser interferometry (modification of dilatometric method) [2]. Photocompositions were placed under covering medical glass (with the aim to prevent the inhibitive effect of oxygen). The UV-irradiation of the system was performed by means of a mercury-quartz lamp DRT-400.

During the experiments the current (Н) as well as maximal (Но) shrinkage of polymerizing composition layer have been determined. The ratio between the current shrinkage and the maximal one was accepted as the relative integral polymerization depth (conversion): Р = Н/Н The kinetic curves of stationary copolymerization were obtained in co-ordinates: conversion vs time.

3. Results and Discussion

3.1. Kinetic Regularities

of Copolymerization of Monofunctional

Methacrylates till High Conversions

Kinetics of photoinitiated copolymerization of monofunctional monomers HEMA and GMA to high conversions was studied at molar ratio of components 4:1, 2:1, 1:1, 1:2 and 1:4. For every system kinetic curves were built at two concentrations of photoinitiator (1 and 2 mol %) and at three intensities of UV-irradiation (7, 17 and 48 W/m2). In Figs. 1-2 the integral kinetic curves of methacrylates copolymerization and their differential anamorphoses are presented.

All integral kinetic curves, regardless of the composition of the polymerizing system and conditions of

1,0

P

0,8 0,6 0,4 0,2

0,0

1,0-P

0,8­0,6­0,4­0,2­0,0

- HEMA 4:1

2:1

- 1:1

1:2

•• 1:4 ■ GMA

50        100       150       200       250 fs 300

-----HEMA

4:1 - - 2:1 --- 1:1 ■■■■ 1:2

......1:4

-----GMA

50        100       150       200       250 300

dP/dt,s1

0,030-

0,025­0,020­0,015­0,010­0,005 0,000

- P0=0.64, W0=0.029 s-1, HEMA P0=0.60, W0=0.022 s-1, 4:1

P0=0.60, W0=0.021 s-1, 2:1

- P0=0.58, W0=0.018 s-1, 1:1 P0=0.61, W0=0.016 s-1, 1:2 P0=0.62, W0=0.013 s-1, 1:4

- P0=0.64, W0=0.010 s-1, GMA

50 100 150 200 250 t s 300

a)

dP/dt, s-1

0,035-

0,030­0,025­0,020­0,015­0,010 0,005 0,000

! ■■      ,   -- P0=0.66,

, .i ;--- p0=0.68,

P0=0.56, W0=0.030 s-1, HEMA P0=0.67, W0=0.025 s-1, 4:1 P0=0.66, W0=0.022 s-1, 2:1 P0=0.70, W0=0.018 s-1, 1:1 W0=0.013 s-1, 1:2 W0=0.009 s-1, 1:4

W0=0.012 s-1, GMA

50 100    150 200 250 300

t, s

b)

0

0

1,0

P

0,8 0,6 0,4 0,2 0,0

- HEMA

■ 4:1

2:1

- 1:1 • 1:2 •• 1:4

GMA

dP/dt s

0,04­0,03 0,02 0,01

0,00

- P0=0.65, W0=0.038 s-1, HEMA

P0=0.62, W0=0.028 s-1, 4:1 P0=0.57, W0=0.023 s-1, 2:1

- P0=0.65, W0=0.022 s-1, 1:1

P0=0.73, W0=0.017 s-1, 1:2 •• P0=0.75, W0=0.012 s-1, 1:4

P0=0.75, W0=0.015 s-1, GMA

200 t,s 250

200 ( s 250

c)

Fig.1. Integral kinetic curves and their differential anamorphoses of photoinitiated copolymerization of HEMA GMA system depending on its composition (IRGACURE 651 2.0 mol %, Т = 293 К) a) Е = 7 W/m[1]; b) Е = 17 W/m[2]; c) Е = 48 W/m[3]

dP/dt, s-1

0,04-

1.0-1

 

 

P

,._:--r-r-'=."

 

0.8-

■'" • / ; / /

/

0.6-

.'      і   . • : і      і   і : : ; //•,''

і

' -----HEMA

 

і •

і .'

■■ 4:1

0.4-

і     ' '. / У

---2:1

 

!  </:■■,■■

----1:1

 

 

.....1:2

0.2-

 

.......1:4

 

: //Л''

------GMA

0.0-

 

 

0,03

0,02

0,01

0,00

50

100

150

200

250

t, s300

P0=0.67, W0=0.039 s-1, HEMA ■■   P0=0.65, W0=0.032 s-1, 4:1 ■ - P0=0.69, W0=0.028 s-1, 2:1

P0=0.72, W0=0.020 s-1, 1:1 ■■ P0=0.67, W0=0.018 s-1, 1:2

P0=0.76, W0=0.012 s-1, 1:4

P0=0.71, W0=0.014 s-1, GMA

50 100 150

200 250t, s300

Fig. 2. Integral kinetic curves and their differential anamorphoses of photoinitiated copolymerization of HEMA GMA system depending on its composition (IRGACURE 651 1.0 mol %, Т = 293 К, Е = 48 W/m[4])

the carrying out process, are typically S-shaped and consist of the protracted, practically linear initial part, the short one that reflects the intensive process of autoacceleration and the protracted part that reflects the slow process of autobraking. However, with increasing of the ratio HEMA:GMA appropriate diminishing of the beginning of the autoacceleration stage conversion P°, conversion Pо at maximal copolymerization rate Wo during autoacceleration stage, and increasing of W are observed.

These results show that it is possible to describe kinetics of monofunctional monomers copolymerization in terms of conception of three reaction zones taking into account the features of copolymerization in every reaction zone.

3.2. Main Principles of the Kinetic Model

1. Polymerizing system is homogeneous that is monomer-polymer phase (MPP) up to the conversion P*, at which solubility of comonomer in monomer solution is maximal. The process of copolymerization takes place in it after the classic kinetic scheme with square chain termination:

chain propagation

A + R     kpij > R

i = 1,2 :

i, , P £ P * (1)

chain termination

'" > product, i = 1,2 .

P £ P * (2)

Specific copolymerization rate Wvi of z-monomer

may be described using the well-known Mayo-Walling equation

= (£pA, [A«][5] + kpl]kp]l[Avl][A,] ])v,n"[6]

{ktll (kpV[])[7] + 2ktvkpi]kp]i [][Av] ] + k]]](kpfl [])[8]}1/:

(3)

where   [Avi ] is i- monomer concentration in MPP.

According to (3) copolymerization rate ((Ovi + (Ov]) must be the quenching function of time, while experimental kinetic curves at this part are practically linear. Probably, with increase of polymer concentration in MPP and respective increase of its viscosity the rate constant k of the square chain termination decreases due to the growth of contribution of macroradicals diffusion braking.

0

50

100

150

0

50

100

150

monomer phase (PMP) precipitates from MPP in the form of nuclei, that is a solid solution of monomers in copolymer. After spontaneous formation of PMP nuclei oversaturation of MPP with copolymer practically disappears, that is why subsequent copolymerization process is accompanied only by the growth of PMP nuclei.

Appearance of the polymer-monomer phase creates two new reaction zones: a volume of PMP and interphase layer at MPP and PMP interface. Let us designate reaction zones volumes as v , v and v , and their volume fractions regarding to the general volume vo of the system - (pv, (ps and (vs, respectively. The volume vs and, respectively, the volume fraction ( vs of interphase layer are complicated functions of the processes of formation, growth and aggregation of PMP micrograins. At the stage of micrograins growth, when fv >>(s, it is possible to expect (vs ~ (s ; at the stage of aggregation, when (pv << (s, on the contrary, (vs ~ (v. In general case, in view of probability of contact between white and black balls in a vessel, it is possible to approximate the connection between ( vs, ( v and ( s by the function

where the proportion coefficient Fvs depends on fractal characteristics of micrograins (both PMP and MPP), their number and the thickness of interphase layer [9, 10].

It is possible to assume, that the volume of the interphase layer is sufficiently small and the condition q>vs << q>v + фs takes place so that (v + (s @ 1. Then it is conveniently to consider fs as the only variable and to rewrite correlation (4) in the form of:

3. In the microheterogeneous system at Р > Р* and t > t* the specific rate of i-monomer expense is determined as the contribution of every reaction zone:

44]

dt

Wvi (1 - (s ) + WvsiFvs (1 - фs )Vs + (Wsi )9s (6)

On the contrary to Q)vi and Q)vsi that are specific copolymerization rates on z-component in volumes of MPP and interphase layer which are homogeneous, <msj> is the specific rate of copolymerization on z-component in micrograins of PMP, averaged by their volume and times of precipitating of PMP portion.

4. Interphase layer at MPP and PMP interface differs from MPP by considerably smaller segmental and, respectively, transmission mobility of macroradicals, which increases the contribution of the diffusion braking into square chain termination with respective abrupt decrease of its rate constant. Nevertheless, let us accept [11], that square chain termination remains dominant, and the kinetic scheme of copolymerization in interphase layer keeps its classic form (1) and (2). Consequently, specific copolymerization rate on z-monomer in interphase layer may be described using Mayo-Walling equation (3) with substantially changed kp and especially kf

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