Crystal nucleation and growth in {poly(_e63-caprolactone) [poly epsilon caprolactone] studied by fast scanning differential calorimetry [Elektronische Ressource] / vorgelegt von Evgeny Zhuravlev

Polybook

Informations

• Publié par: universitat_rostock
• Publié le: 01 janvier 2010
• Nombre de lectures: 15
• Langue: English
• Poids de l'ouvrage: 21 Mo

Extrait

Crystal Nucleation and Growth in
Poly(-caprolactone)
Studied by Fast Scanning Differential
Calorimetry

Dissertation

zur Erlangung des akademischen Grades
Doktor der Naturwissenschaften
Dr. rer. nat.

am Institut für Physik
der Mathematisch-Naturwissenschaftlichen Fakultät
der Universität Rostock

vorgelegt von
M.Sc. Evgeny Zhuravlev
geboren am 30. September 1983 in Saransk
aus Russland


Rostock, Juni 2010
urn:nbn:de:gbv:28-diss2010-0181-1












Gutachter:
Prof. Dr. Christoph Schick Universität Rostock, Institut für Physik
Prof. Dr. Eberhard Burkel
Prof. Dr. Toshiji Kanaya Kyoto University, Institute for Chemical Research

Tag der Disputation: 23.09.2010

Content
1. Introduction.................................................................................................................... 4
2. Literature review............................................................................................................9
2.1. Polymer crystallization.............................................................................................9
2.2. Thin film fast scanning calorimetry ....................................................................... 21
3. Fast scanning differential calorimeter with power compensation................................ 32
3.1. Issues of single sensor calorimeter......................................................................... 32
3.2. Differential temperature control scheme with power compensation ..................... 34
3.3. Electric scheme and power difference determination ............................................ 39
3.4. Hardware and software realization 42
3.5. Solidification of metals studied by fast scanning calorimeter................................ 45
3.6. Temperature calibration.........................................................................................48
4. Heat capacity determination 54
4.1. Scheme for differential power determination......................................................... 54
4.2. Heat Balance..........................................................................................................55
4.3. Determination of differential loss function ............................................................ 58
4.4. Crystallization of polyethylene (PE)...................................................................... 62
4.5. Solidification of pure and nucleated poly(-caprolactone) (PCL) ......................... 64
5. Crystal nucleation and overall crystallization kinetics in PCL .................................... 67
5.1. Influence of existing nuclei and crystals on subsequent heating ........................... 67
5.2. Elimination of homogeneous crystal nuclei formation in PCL on cooling............ 71
5.3. Annealing experiments with PCL .......................................................................... 76
5.4. Characteristic time of nucleation and crystallization at different temperatures..... 79
6. Discussion.................................................................................................................... 83
7. Summary...................................................................................................................... 91 Content 3
8. References.................................................................................................................... 92
Appendix ............................................................................................................................ 100
A1. Experimental data for all heating curves after annealing..................................... 100
A2. Fast scanning calorimeter software ...................................................................... 111
A2.1. Measurement software.............................................................................111
A2.2. Data evaluation.........................................................................................117


1. Introduction
Solidification from the melt has been actively studied since ancient times. The process
of metals casting, glass blowing and injection molding of plastics are all basic examples of
shaping a liquid in order to produce solid parts.
In modern times, people have been trying to modify these procedures to improve the
properties of the product, to reduce manufacturing costs and to make production more
environmental-friendly. Special heat treatment is an important constituent of this type of
technological processes. An example of such treatments is quenching. In metallic systems, it
is commonly used for hardening the materials or in order to avoid phase separation in alloys.
Extremely rapid cooling of metallic melts can prevent the formation of crystalline structures.
As the result, amorphous vitreous metals are formed, which are of high practical interest
nowadays. Very rapid cooling is also used in polymer production to avoid high temperature
phase transitions providing a possibility to reduce the degree of crystallinity which results in
an increasing toughness of the materials. The critical cooling rate which is needed to make a
material amorphous is an important parameter not only for polymers. The ability to quench
materials without crystallization allows one performing of crystallization at desired
temperatures during a special heat treatment in subsequent heating. This method is used in
production to control the crystallization rate in order to modify properties of the final product.
The common way of treating crystallization consists of two different basic processes;
critical nuclei formation and their subsequent growth. Same can also be applied to polymers.
Nucleation, i.e. the stochastic formation of nuclei of the new phase capable to a further
deterministic growth, is an essential ingredient of crystallization. However, the direct
measurement of nucleation is not yet possible, as a rule, by the existing techniques. This is
due to a very small size of the nuclei and very small effects (e.g. heat effect) which occurs in
the course of their formation. Therefore indirect methods are usually utilized in order to detect
them. One of them is known as polarized optical microscopy.
In polarized optical microscopy, the number of supercritical nuclei formed can be
determined by counting the number of finally formed crystals and identifying both numbers.
However, the optical microscopy technique is limited in its application to materials where the
crystals formed and matured are visible and that have slow enough crystallization rates. In 1. Introduction 5
calorimetry, the enthalpy of cold crystallization on heating was found to be dependent on the
number of previously formed nuclei. As cold crystallization, we denote hereby the
crystallization processes occurring at heating the sample from below the glass transition
towards the melting temperature. Mathot et al. [1] observed a reduction of the cold
crystallization peak on heating after cooling with increasing cooling rate. But, even a high rate
differential scanning calorimeter (DSC) was not able to prevent fully the nuclei formation on
cooling in poly(l-lactic acid) (PLLA), a rather slowly crystallizing polymer. Oguni et al. [2]
and Vyazovkin et al. [3] studied by similar methods nucleation below the glass transition
temperature in low molecular mass organic substances by analyzing data on crystallization on
heating after annealing at different temperatures for different times. In this way, DSC may
serve as an effective tool to study nucleation processes by analyzing the heat effects.
However, most of the existing studies are investigating either crystallization (i.e.,
nucleation and simultaneous growth) or nucleation in different ranges of temperature. One of
the reasons for that is the separation of these two processes in time and temperature. Figure
1A shows the results of several separate measurements of crystal nucleation and growth
kinetics in glycerol (taken from [4]).

A B
Figure 1. Temperature dependence of the steady-state nucleation rate J and of
linear rate of crystal growth for glycerol from [4] (A). Schematic representation of half-time
of nucleation and crystallization versus undercooling temperature (B).
1. Introduction 6
The full separation of these two processes, as shown in Figure 1A, is not always the
case and in different materials they can overlap, see Figure 1B. In this case the study of both
processes at the same temperature becomes important. In Figure 1B the nucleation and overall
crystallization half-times are represented versus undercooling below melting temperature. The
shadows below the curves show the distribution of the corresponding processes around their
characteristic times. The width of these regions is commonly several orders of magnitude in
time. Here a time scale is assigned based on the result of this work showing poly(-
caprolactone) (PCL) crystal nucleation and overall crystallization kinetics. For investigation
of nucleation process at least the experiment at maximum nucleation rate (minimum
nucleation half-time) is of interest. For simultaneous determination of crystallization half-time