Film Formation and Mechanical Behavior of Polymer Latices

Chapter 10

Film Formation and Mechanical Behavior of Polymer Latices 1

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1

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Downloaded by NORTH CAROLINA STATE UNIV on October 14, 2012 | http://pubs.acs.org Publication Date: October 15, 1996 | doi: 10.1021/bk-1996-0648.ch010

C. Gauthier , A. Guyot , J. Perez , and O. Sindt 1

Groupe d'Etudes de Metallurgie Physique et de Physique de la Matihre, Institut National des Sciences Appliquées, 20 avenue Albert Einstein, F-69621 Villeurbanne, France Laboratoire de Chimie des Procedes de Polymerisation, CPE Lyon, B.P. 2077, F-69616 Villeurbanne, France 2

The aim of this work is to obtain relevant information on the film forming process in order to control the development of mechanical properties of the latex film. As appropriate mechanical behavior cannot be reached i f coalescence is not achieved, we first investigate the coalescence of the core-shell latex by performing weight loss measurements at different temperatures and humidities. After a brief survey of the literature, we propose a model of coalescence with a new description of the deformation of latex beads, based on the diffusion of structural units under the polymer / water interfacial forces. Calculations based on this model suggests that particle deformation and water evaporation may display different kinetics. Therefore, the end of coalescence may be related to the slowest process. At the end of coalescence stage, mechanical behavior of the film can be investigated. Then, we perform tensile tests for different time of annealing during the autohesion phenomenon. During this film forming stage, the behavior of the film changes drastically from brittle to ductile. We observe two distinct evolution for strain at break and stress at rupture. The former is attributed to the formation of entanglements through the particles boundaries whereas the latter might concern water diffusion throughout the film. The process consisting of driving a latex from its colloidal form to a continuous film needs a good understanding since it is of great importance for industry. Film formation is a critical aspect of all applications that involve coating a surface or forming a layer with good cohesive properties. Consequently, great efforts have been devoted to the study of this phenomenon. Several stages during film formation have been observed experimentally and a phenomenological description of the process, divided into three parts, is generally accepted (1) : 0097-6156/96/0648-0163$15.00/0 © 1996 American Chemical Society In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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FILM FORMATION IN WATERBORNE COATINGS

Stage 1- Water evaporation and colloid concentration. As water evaporates, a uniform shrinkage of the inter-particle distance occurs and the voids are gradually filled by particle sliding, until a dense packing of spheres is obtained. Stage 2- Particle deformation and evaporation of the bounded water (i.e. water perturbed by the high concentration of surfactants at the particle surface). This stage, also called coalescence, results in a honeycomb like structure of deformed particles in dodecahedra. Stage 3- Interdiffusion of macromolecules. In this stage the mechanical strength increases and the water permeability of the film decreases. Under certain conditions, the polymer chains can diffuse through the particle boundaries. The honeycomb like structure disappears and an homogeneous, continuous film is formed. These three different stages have been investigated both theoretically and experimentally. Much of the early work on latex film formation focused on the early stages of the process: water evaporation accompanied by particle concentration followed by the particle coalescence and deformation into dodecahedra. Various models have been proposed to describe the process of film formation. They will be briefly reviewed in the first part of this paper. Considering some of the main arguments of the theories which were previously established, a physical model that describes the coalescence process is advanced. This model will be confronted to our experimental data obtained from weight loss measurements at various temperature and humidity. Then, the evolution of mechanical behavior of latex films during stage 3 will be studied. Mechanisms and Theories for Film Forming Process : a survey From the standpoint of thermodynamic, the process of coalescence of polymer droplets into a film is favourable because of the decrease in free energy achievable with minimization of total surface. A l l contiguous particles would flow into deformed spheres representing a minimization of surface and gravitational energy. During stage 1 of film formation, as water evaporates from the surface, the particles centers approach each other but particles remain separate until they are forced into contact by spatial limitations. The second stage begins when the particles can no longer slide each other into new positions. Then, particles are brought into close proximity so that on can assume that their stabilizing layers may collapse, resulting in polymer-polymer contact. Coalescence Stage. In a first attempt to propose a coalescence criterion, Dillon et al. (2) stated that particle coalescence resulted from the viscous flow of polymer induced by the surface tension between water and polymer particles. Nevertheless their theory could hardly describe the film forming process of crosslinked particles (stated on viscous flow only). A few years later, Brown (3) noted the influence of the water evaporation rate and concluded on its driving role. He proposed to make a force balance and, by neglecting some parts of his equation, concluded that capillary forces were driving coalescence (taking into account the water/air surface tension). This

In Film Formation in Waterborne Coatings; Provder, T., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


10. GAUTHIER ET AL.

Mechanical Behavior of Polymer Latices

165

model has been completed by Mason (4) and Lamprecht (5) who improved the mathematical description. Vanderhoff et al. (6) argued that capillary forces calculated by Brown were too low to insure coalescence of particles bigger than 200 nm in diameter. They proposed to take into account the driving forces arising from the polymer water interfacial tension. They used the mechanism stated by Dillon et al. (2) and concluded that capillary forces were driving coalescence at the beginning, but after a certain threshold, the intra-particle pressure caused by the polymer/water surface tension provoke coalescence. This concept of capillary forces and interfacial forces acting in tandem to promote latex coalescence has been developed by Eckersley and Rudin (7) . They compared the values of deformation of the latex beads related to the radius of the circle of contact of the spheres in both theories. In order to fit the experimental observations, they proposed to add the contributions of both forces. Sheetz (8) described the process as follows : under water/air surface tension, the beginning of the film formation is caused by capillary forces. These forces are not only perpendicular to the film's surface, but a component parallel to the surface exists. The perpendicular part of the force can be described by Brown's equations. But, under the parallel force action, a rapid close of the capillary takes place at the surface of the film, resulting in a lower water evaporation rate. Since the remaining water has to diffuse through the surface of the polymer film, one should consider that the free energy of water evaporation is transformed into mechanical compression work that ensures the coalescence. Most of these analyses describe the deformation of the latex particles using the Hertz contact solution for the deformation of two elastic spheres that were pressed into contact. Since the polymer is in fact viscoelastic, the error may be compensate by replacing the elastic shear modulus G by a time dependant shear modulus G(t). Lamprecht (5) and Eckersley and Rudin (7) have proposed to introduce the mathematical development generating the equivalent viscoelastic solution to the Hertz contact problem. Recently, Sperry et al. (9) proposed comparative examinations of the compacting stage in the absence as well as in the presence of water. Int is generally admitted that particle deformation occurs only above the minimum film formation temperature (MFT) which often lies close to the glass transition temperature (Tg) of the latex polymer. MFT is defined to be the minimum temperature at which a latex cast films becomes clear . Below this temperature , the dry latex is opaque owing to interparticle voids. These authors have observed that a film pre-dried well below the M F T displays a transition from turbid to clear film as it is heated. They were convinced of the lack of a special role for water in this process and proposed to decompose it into separate water evaporation and film compacting events. The Third Stage of Film Forming Process. Once the second stage has correctly occurred, the so-called stage 3 can begin. In this final stage, interdiffusion of polymer chains of adjacent particles across particle boundaries causes further coalescence. Several studies have been published on direct non radiative energy transfer (DET) using fluorescence decay measurements (10-11). These measurements required to



166


label particles with appropriate donor and acceptor dyes. Small angle neutron scattering (SANS) is also a technique which provides an effective way to study the third stage. It measures the growth in particle size of deuterated latex particles as selfdiffusion progresses. For the first time in 1986, Hahn and co-workers (12) reported work on coalescence study by SANS on butyl methacrylate (BMA) polymers. Linné et al (13) published SANS measurements on the actual interdiffusion depth in polystyrene. From these data, it results that the molecules originally constrained to ~ 160 Â in the latex expand to ~ 660 Â as the polymer is annealed above glass temperature to form a polymer film. They highlight that the molecular weight and the location of chain ends are important parameters of polymer interdiffusion process. To obtain quantitative information, one should fit experimental data to a model describing the polymer diffusion. Two different approaches are found in literature. One assumes that polymer diffusion can be evaluated by using models based on the Fick's second law. One the other hand, different theories based on De Gennes reptation diffusion model were developped. They describe the development of material strength, due to the formation of entanglements between polymer chains across the interface. Along this second line, four theoretical models have been proposed (14-17) ; they are reviewed and compared to experimental SANS data in (18) . These models agree that fracture stress should increase with healing time to the one-fourth power whereas a one-half power law is deduced from Fick's second law. Different studies (17-18) present analysis based on these models. Nevertheless, considering the whole literature, no evidence is driven from experimental results to invalidate one or the other approach. Besides SANS experiments, Yoo et al. (18) measured the tensile strength of polystyrene films and reached the conclusion that penetration depth for full mechanical strength may be partially controlled by the spatial distribution of chains ends in the interface and the dimensionless ratio of polymer chains' radii of gyration to the latex particle's radius. From their experimental results, the order of magnitude for the minimum penetration depth, that gives a cohesive film, is around a few nanometers. The development of film strength has been also studied by Zosel et al. (19) with poly (butyl methacrylate) (PBMA) . Tensile tests were performed after increasing annealing time at 90°C. The tensile behavior changed completely from brittle to ductile within the first five minutes and the corresponding penetration depth measured by SANS lies once again in the range of a few nanometers. Finally, fracture energy seems to rise to a constant value when the interdiffusion depth has the same order of magnitude as the radius of gyration of the P M B A molecules. In conclusion, effective correlation can be obtained from comparison between SANS measurements and mechanical tests in the third stage of film formation process. Nevertheless, direct studies of the third stage requires heavy equipments and specific modifications of the latex samples. In this study, we are looking for relevant information on the film forming process in order to control the development of mechanical properties of the latex film. Our investigations were first devoted to stage 1 and 2 since appropriate mechanical behavior can not be reached if coalescence is not achieved. In the second part of this work, the third stage of film forming process is analysed through evolution of tensile behaviour of the latex film with increasing time. 1


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Experimental Numerous parameters should be considered in order to understand the film forming process. It is generally accepted that these parameters can be divided into two parts. The first one is related to latex properties such as particle diameter, glass transition and viscoelastic properties of the polymer , the nature of surfactants, etc. . The second set concerns the physical parameters of the film forming process and it includes temperature, humidity, the interfacial energy (either water/air or water/polymer). In this study, the parameters of the latex are kept constant and we focused our attention on physical ones. Latex Preparation. Structured core-shell lattices are prepared by seeded emulsion polymerization. Homopolymerization of polystyrene seed is first performed in batch using Sodium Dodecyl Sulfate surfactant (SDS). The size of the seed particles is 100 nm in diameter. In a second stage, polymerization of styrene and butyl acrylate is achieved by a semi-continuous process. The shell composition is 50 weight % Styrene - 49 weight % Butyl acrylate with a low content (1 weight %) of acrylic acid. The final size of particles is 200 nm. The feed rates of monomer and the initiator concentration are adjusted in order to avoid any composition drift during the shell polymerization. Moreover, a parallel continuous addition of SDS water solution is performed in order to maintain the stability of the particles during and after polymerization. We check that no nucleation of new particle population occurs, and that the number of particles is constant and equal to the number of seed particles. Film formation is obtained after dehydration of the latex and maturation in a regulated oven (temperature and humidity) . During film formation, the core-shell particles deform into a film with a soft matrix including hard spherical inclusions of poly (Styrene) (Tg « 100°C). Glass transition temperature of the soft matrix has been determined by DSC measurements and stand around 20°C (heating rate : 10°/min). Weight Loss Measurements. The film forming process is studied by weight loss measurements. A Teflon mould is filled with the latex and weighted on a balance in a regulated oven. Data are recorded on line by a computer . The weight loss curves versus time are recorded for different temperature and humidity conditions. On these curves, the three stages of film forming process can be determined (Figure 1) : Stage 1- Linear decrease of the mass with time; the bulk water evaporates resulting in latex concentration at a rate depending only on temperature, hygrometry and electrolyte concentration in the water phase. Stage 2- Evaporation continues and particles are emerging at the surface of the film. The rate of evaporation decreases; coalescence occurs and the honeycomb like structure is formed. Stage 3- No important variation of mass with time; only polymer diffusion can occur. The times for stage 1 and stage 2 are measured as well as the water evaporation rate in the stage 1 (from the slope of the curve). Experiments are carried maintaining


168


all parameters except of temperature and humidity constant (air velocity, mould size, latex weight) Tensile tests. The mechanical properties of latex films at large strains and their fracture behaviors have been studied in uniaxial elongation by means of a commercially available tensile tester (Instron 8561). Tensile tests are performed at 23 °C after various times during stage 3 in order to obtain nominal stress σ = F/So versus elongation λ = l/lo. Temperature of testing is carefully controlled with maximum deviation of +/- 0.2°C. The dimensions of the specimen (H3 type) are 50x4x1 mm and the cross head speed is 200 mm/min. A minimum of five samples are tested for each experiment. True stress - strain curves are derived from conventional curves using the iso - volume assumption. A series of tensile curves is obtained when film formation is achieved at Τ = 32°C


3

Results Water Evaporation. Considering the weight loss measurements, all the curves first display a linear dependence versus time. Although this first stage does probably not determine the final properties of the film, we believe, considering Sperry's study (9) , that the rate of water evaporation event should be estimated independently of film forming process. Before the beginning of stage 2, when all the mold surface is free, water evaporation is a function of water pressure, humidity ,air velocity and evaporation surface. The evolution of water evaporation rates, respectively, with temperature and humidity are reported in Table I and Table II. At a fixed temperature, water evaporation rate decreases linearly when humidity increases. On the opposite, it increases with temperature when humidity is constant. Table I. Water evaporation rate versus Temperature (Relative Humidity = 75%) T(K) Evaporation rate (mg/min)

283 4.6

293 6.2

301 9

297 8.2

305 9.2

309 9.9

313 10.1

Table II. Water evaporation rate versus Relative Humidity (Temperature 20°C) R.H. (%) Evaporation rate (mg/min)

52 9.8

55 9.2

75 6.2

65 7.7

85 5.2

90 4.8

98 3.7

Water evaporation is constant since the end of stage 1. In the second stage, the process is slowed down. As described in literature (20), during coalescence, water evaporation is not homogeneous on the whole surface of the mould. Nevertheless, we propose to estimate roughly the total time (tj +1 ) which is necessary to evaporate the amount of water ( M ) present in the sample. We decided to calculate two limiting 2

w a t e r


10. GAUTHIER ET AL.


169

values from the value of water evaporation rate obtained from the slope of the linear decrease of weight during stage 1. In a first assumption, we fixed a water evaporation rate equal in stage 1 and stage 2. *

,

_

f

M ter wa

M+ h - — rate p j


eva

0rat

on

It seems quite evident that this calculated value overestimates the real one since it neglects the variation of the surface of evaporation. Hence, a second limit is calculated taking into account the variation of water evaporation rate due to the decrease of free surface. We consider that at the end of stage 1, polymer particles with a defined a radius are emerging and that water evaporation occurs only in the interstitial regions between spheres in close packed array (Scheme 1). ο

free_

13

Ç

^ total

~

Q

hexagon Q

_ Q

particle

^hexagon

Calculations have been made for each experiment varying temperature and humidity. For example, at T= 40°C and R.H. = 75% , the time for water evaporation during 2 * stage is 60 min as calculated with assumption 1 whereas experimental time for coalescence is about 150 min. Assumption 2 leads to t = 800 min. For any temperature and hygrometry, experimental data are in the range of the two calculated limits. The results related to temperature are reported on Figure 2. However, one could be surprised by the fact that the measured times are always nearer the values obtained with the first assumption, which seems in a first view, quite unrealistic. Thus, since time to evaporate water is well approximate with the first assumption, there should be a mechanism occurring during stage 2 which maintains a high level of free surface for water evaporation, higher than the only interstitial surface. We consider that this mechanism consists in particle deformation, which is assumed to control the coalescence. nc

2

Physical Model for Coalescence Mechanism. The calculation we proposed first considers that coalescence process is governed by the value of compacity. At the beginning of stage 2, compacity of the latex is 0.74. which corresponds to a closepacked array of spheres. At this point, latex particles are emerging at the surface. In fact, the whole coalescence stage will consist in an increase of compacity from 0.74 (close packed array) to 1 In order to obtain a theoretical model for coalescence, one should first define the driving forces acting on latex particles. From our point of view, the two mechanisms , called by Rudin capillary force or polymer water interfacial forces, are the expression of the same physical principle. Practically, both forces are calculated from Young and Laplace law, one considering the water / air interface (capillary force) and the second the polymer / water interfaces. Capillary forces lead to restrict the mechanism to the surface of the mould which can not be entirely true in macroscopic samples, as already discussed by Sheetz. So, our description will take



170


Scheme 1 : Surface evaporation.



10.

171


GAUTHIER ET AL.

into account the driving force derived from the Young- Laplace law using the polymer / water interfacial energy. Thus, the assumptions of our model are that the Young and Laplace law describes the driving forces for coalescence which are opposed by viscoelastic deformation of the polymer. In adequate conditions (at temperatures above the glass transition temperature) polymer diffusion is involved. In this model, the calculations are made in terms of polymer diffusion whose description is based on the formal statement proposed by Nabarro-Herring in order to describe the high temperature creep of polycrystals (21). The system is presented on Scheme 2. We have to consider two distinct points: A in the zone between two adjacent particles (called coalescence zone) and Β somewhere in the particle. Due to the difference of curvature between the coalescence zone and the rest of the particle, a difference of pressure exists between A and Β which could be minimized i f the curvature radius of the coalescence zone (called r) decreases. This induces polymer diffusion from the particle to the coalescence zone. In the absence of other particles, the system tends to form one bigger particle. In the close-packed latex, particles will be transformed into dodecahedra. Calculations are made only on two particles considering the symmetrical aspect of the problem. In fact, some refinement should be obtained taking into account gravitational force which introduces some anisotropy. The pressures in points A and Β are derived from Young and Laplace equation. Pressures are function of the polymer / water interfacial energy and are of opposite signs.

A

r

R

Also, some geometrical considerations enable us to establish a relation between r, R and a (the half of the height of the coalescence zone) : 2 2

2

(a + r) + R = ( R + r )

2

a 2(R-a)

2

a 2R

The driving force related to the difference of pressure can be expressed by the following relation where μ is the chemical potentiel of a structural unit of the macromolecule and v the volume of a structural unit. 0

F = -gradμ In point A and Β :

μ =μ +P .v

Then ,

F = -gradμ = - ^ '

Α

0

A

μ =μ +P .v

0

Β

V

0

B

0

°


172



2500 \

• >

Experimental Lower limit

\

2000

s

- - - Upper limit

s

2

Q

m

1500

+

•

& 1000

500 20

30

40

Temperature (°C)

Figure 2 : Duration of Stage 1 and Stage 2 versus Temperature at 75% Relative Humidity.

Scheme 2 : Coalescence mechanism : description of the system.


10. GAUTHIER ET AL.


173

Using the formal statement of Nabarro-Herring (21) , we can relate this force with the diffusion of polymer structural units via the rate of diffusion ( V ^ f ) where D = diffusion parameter, k = Boltzmann constant :


V v

d i f

. -- ™ k T

From the rate of diffusion, we can deduce the diffusion flow (J). By définition, it is the number of structural units which cross a unitary surface per second and corresponds to the rate of diffusion divided by the volume : V

j- dif.

v

DF

=

0

"kT*

a 'lr

D

(PA-PB)

kT

a

=

v kT

0

+

Rj

kT* a ' l a

2

+

The diffusion flow leads to the growth of the volume v zone by diffusion of structural units through its surface π a :

Rj c o a

| of the coalescence

2

^W

=

J V o K a 2

dt v

coa

=

2a

Dvta

0

kT

R

4Rl +

a J

j can be calculated once again from geometrical considerations : v

2

coal = ™ . e

e = rcosO = dV

coal da

2 =

*a R

2

rR

a

r +R

2R

3

Therefore the derivative expression of a on time: da _ 2 π Ρ σ ν ρ Κ

aA

ο

dt ~

kT

+

l a

R

R j ^ a

3

Then, we can express the variation of a(t), height of the coalescence zone : a ( t ) = j ^ , kT

1

2R^

dt

The expression of a(t) takes into account the following parameters : R (particle radius) σ (water/polymer interfacial energy » 35 mJ/m ) and vo (volume of the 2


174


structural unit). As the length of a structural unit of the polymer chain λ is about 5 Â, the volume of the structural unit (vo » ) can be estimated as 125 . In this description, the diffusion coefficient D can be related to the time necessary to move a structural unit over a distance of its own length (called x | ) · The determination of this time is explained elsewhere (22) . mo

T m o l

λ

2


*mol(T)

The coalescence time (stage 2) is obtained when the spherical particle is deformed into a decahedron. That corresponds to a value of a(t) at the end of stage 2 which is about 55 nm whereas our particles have a radius of 100 nm (see Scheme 1). Results and Discussion. A l l results are reported in Figure 3. Calculated times for coalescence (line) are compared to experimental data obtained from the weight loss measurements (dots). We also plot the calculated times for water evaporation as calculated in the first assumption. Two distinct zones are observed. Above 300 K , the calculated times for coalescence are lower than measured values. In this region, particle compaction is very easy due to the low value of polymer elastic modulus meaning a rapid diffusion of polymer units. Although particle deformation can be very fast, complete coalescence is not reached since films still contain lot of water. Moreover, in this range of temperature, we find that experimental data are close to calculated times for water evaporation (obtained from experimental water evaporation rates). Therefore, we can consider that water evaporation is the governing phenomenon in this range of temperature. As proposed by Sheetz (8) , there is probably a rapid close of the capillary at the surface of the film, resulting in a lower water evaporation rate because water has to diffuse through the surface of the polymer film to evaporate. On the contrary, below 300 K , the prediction of particle deformation event is longer than measured data from weight loss curves. In this range, particle deformation becomes more difficult as temperature moves closer to Tg. Moreover, one could argue that, since no more water is present in the film, calculations should be revised after t2, taking into account the polymer / air interfacial energy . In fact, this may only lead to increase even more of the calculated times. Below 300 K , we observe that experimental data are not any more close to calculated values for water evaporation. To our point of view, this means that, even if the calculated curve does not fit exactly the experimental data, we can consider that deformation is the phenomenon which controls the kinetic of film formation. In this range of temperature, the films produced can contain nano voids, corresponding to the uncomplete filling of interstitial domains. In conclusion, in agreement with Sperry's results, we think that deformation of particles and evaporation events have different kinetics. Consequently, the time for coalescence is related to the slowest process. For our samples, at T>300 K , the slowest event is water evaporation. Below this temperature, the deformation of particles determines the end of coalescence. From a practical point of view, the choice of film forming conditions should take into account this aspect. So, we decide to adopt a film


10.

GAUTHIER ET AL.


175


forming temperature so that both phenomena display comparable times. Further studies are performed with a film forming temperature of 32°C. Development of mechanical strength during film formation. Since no information can be obtained about the third stage from weight loss curvs, mechanical properties were investigated. We decide to study the influence of annealing time during autohesion on the tensile properties of the films. At 32°C, the time for evaporation and compaction processes are similar. In this case, we can without any problem define the initial time for stage 3. The results of tensile tests which were performed at 23°C after different annealing times at 32°C are plotted on Figure 4. During the third stage, the behavior of the material changes drastically with increasing times. At low annealing times, the film displays a brittle behavior with a strain at break (ε ) lower than 0.1. The behavior changes from brittle to very ductile for long annealing times and a plastic consolidation domain can be observed. The strains at break become higher than 5. In fact, this drastic change results from two different types of evolution of the mechanical properties. For short annealing times, the stress at break increases first, and then (one decade later) the strain at break goes up quickly to a plateau value. For times longer than 500 min, the strain at break stabilizes (Figure 5). One the contrary, for the same time of scale, the plastic stress still increases slowly (Figure 6). The rapid change of tensile behavior from brittle to ductile has been observed by Zosel et al. (19). The fast increase of the film toughness has been interpreted in terms of progressive interdiffusion and formation of entanglements of the macromolecules. In our opinion, (i) the increase of stress shows formation of Van der Waal s bounds between particles, resulting in the cohesion of the whole material and (ii) the increase of the strain at break is due to the diffusion of macromolecules through particles boundaries which increases the number of entanglements between two particles. Moreover, the slow increase of plastic stress for longer annealing times can be related to a redistribution of the macromolecules in the film which increases intermolecular Van der Waal's interactions. The main cause of such a phenomenon should be the diffusion of water molecules through out the film. This assumption has been checked by more precise water loss measurements and tensile tests on wet and dry films. Due to the annealing temperature chosen by Zosel (T=90°C), the plastic stress evolution due to water diffusion could not have been observed in his experiments. Some confirmations of these results have been obtained by forming the film at another temperature. At 40°C, we observe once again both types of evolution. The time necessary to reach the steady state strain at break is reduced, as the diffusion becomes easier at higher temperature. Moreover, a plateau for plastic flow stress is also reached for annealing time of about 3000 min. In a future work, we propose to establish a correlation between the kinetics of the evolution of the mechanical properties and that of the diffusion of polymer units or of water through the film. Γ


176


• 310

1 - - - Model 1 •

ί

•

305

κ \


g

Water evaporation

!

i

•

Experience

•

285

0

100

200

300

400

500

Time (min)

Figure 3 : Comparison of calculated coalescence times with experimental data.

ε Figure 4 : Tensile test curves after different annealing times during stage 3.


GAUTHIER ET AL.


Temperature = 32°C, R.H. = 75%.


co 4

t

3

h 2

0 10

100

1000

10000

100000

Time (min.)

Figure 5 : Strain at break after different annealing times during stage 3.

Temperature = 32°C, R.H. = 75%

•

•

v> ci

S

2

0 10

100

1000

10000

100000

Time (min.)

Figure 6 : Plastic flow stress after different times during stage 3.


178



Conclusion The film forming process of a classical core-shell latex has been investigated considering the development of mechanical behavior of the film. The analysis of the coalescence stage suggests that particle deformation and water evaporation may display different kinetics. Therefore, the end of coalescence may be related to the slowest process. The selection of film forming conditions should take into account this aspect and we propose to choose a film forming temperature so that both phenomena display comparable times. This could correspond to some optimum conditions in regards to the presence of voids. When coalescence (stage 2) is complete, film displays a cohesion but with a brittle behavior. With increasing time of annealing during the third stage of the film forming process, the behavior of the film changes drastically from brittle to ductile. This evolution is attributed to the formation of entanglements through the particles boundaries whereas the plastic stress of the film may reflect water diffusion throughout the material.

Literature Cited 1. Rios Guerrero., Macromol. Symp. 1990, 35/36, 389. 2. Dillon R.E., Matheson L.A., Bradford E. B., J. Colloid Sci. 1951, 6, 108. 3. Brown G. L., J. Polym. Sci.1956, 22, 423. 4. Mason G., Br. Polym. J., 1973, 5, 101. 5. Lamprecht J., Colloid & Polym.Sci.1965, 9, 3759. 6. Vanderhoof J.W., Tarkowski H.L., Jenkins M.C., Bradford E.B., J. Macromol. Chem. 1966, 1, 131. 7. Eckersley S.T., Rudin Α., J. Coat. Tec., 1990, 62, 89. 8.Sheetz D. P. ,J. Appl. Polym. Sci., 1965, 9, 3759. 9. SperryP.R., Snyder B.S., O'Dowd M.L., Lesko P.M., Langmuir, 1994, 10, 2619. 10. Zhao C.L., Wang Y., Hruska Z., Winnik Μ. Α., Macromolecules, 1990, 23, 4082. 11. Peckan O., Canpolat M., Göçmen Α., Eur. Polym. J., 1990, 29,1, 115. 12. Hahn K., Shuller G., Oberthür, Colloid & Polym. Sci., 1986, 264, 1092. 13. Linne Μ. Α., Klew Α., Sperling H., Wingall G.D., J. Macromol. Sci. Phys. 1988, B27, 181. 14. De Gennes P. G., C. R. Acad. Sci. Paris 1980, B291, 219 15. Prager S., Tirrel M., J. Chem. Phys., 1981 , 75, 5194 16. Jud K., Kausch H.H., Williams J.G., J. Mater. Sci., 1981 , 16, 204 17. Kim Y.H., Wool R.P., Macromolecules, 1983 , 16, 1115 18. Yoo J.N., Sperling L.H., Glinka C.J., Klein Α., Macromolecules, 1991 24, 2868 19. Zosel Α., Ley G., Macromolecules, 1993 , 26, 9, 2221 20. Feng J., Winnick M.A., proc. ACS PMSE Chicago, 1995,90 21. Poirier J.P., in "Plasticité à haute température des solides cristallins" Eyrolles Ed., Paris 1976 p. 152 22. Perez J. in "Physique et mécanique des polymères amorphes" Lavoisier Ed., Paris 1992.


Film Formation and Mechanical Behavior of Polymer Latices

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